Harper's Electricity Book for Boys

The hoop should not touch the base-block, but should clear it by a quarter or half an inch. Make the coil ends fast (as described for the astatic galvanometer and illustrated at Fig. 9) by means of binding-posts. The wires need not be carried over the top of the block, but may run through holes under the hoop and along grooves cut in the under side of the block and leading to the foot of the binding-posts.

The graduated card should be made from a piece of stout bristol-board or heavy card-board having a smooth, hard surface. It is laid out with a pencil or pen compass, as shown at Fig. 12, and should be three inches in diameter. The card is placed on the wood strip or ledge, so that the zero marks will be at the front and rear, or at right angles to the hoop and coils of wire. The compass needle, when at rest, should lie parallel with the coils, so that the current will deflect the needle and send the indicator around to one side or the other of zero, according to the direction in which the current is passing through the coils.

This is more clearly shown at Fig. 13. The circle represents the outside diameter of the card; the dark cross-piece, the magnetic needle; and the pointed indicator, a stiff paper, or very thin brass or copper strip, cut and attached to the needle with shellac or paraffine.

When at rest the magnetic needle should be parallel to the coils. To insure this the instrument must be moved so that the lines of wire forming the coil will run North and South. Otherwise the N-seeking end of the magnetic shaft will point to North, irrespective of the position occupied by the wire coil.

The magnetic needle may be made as described for the compass (see chapter iv., Magnets and Induction Coils). It should be arranged to rest on a brass pivot pressed down into the cross-piece of wood.

The indicator-needle may be cut from stiff paper, thin sheet-fibre, or very thin cold-rolled brass or copper, the latter being commonly known as hard or spring-brass. Only one pointer is really necessary—that pointing to the front. But the weight of the material would have a tendency to upset the magnetic needle, and therefore it is better to carry an equally long tail or end, on the opposite side, to properly balance the needle.

A very weak current, passing in through the first post and out at the third, will cause the indicator to be deflected considerably, or so that it will point from 40° to 60° on either side of the zero point, according to the direction in which the current is running through the coils.

When not in use the magnetic needle should be removed from the pivot, and placed in a box or other safe place, where it will not become damaged.

A differently arranged tangent galvanometer is shown at Fig. 14. As the line of binding-posts would indicate, there are several coils of wire about the circle or hoop.

This galvanometer can be used for either strong or weak currents, since it is wound with both coarse and fine insulated wire. An upright plate of wood, seven inches wide and eight inches high, supports the hoop and compass. The top corners are sawed off, and four inches above the bottom a straight cut is made across the plate, five inches wide and arched in a half-circle five inches in diameter. A shelf of wood a quarter of an inch thick, three inches wide, and five inches long is made, and attached as a ledge in this arched opening, so that a compass three inches in diameter may rest upon it.

The shelf should be arranged so that it will hold the compass in the middle of the circle instead of at one side. The turns of wire will then be in line with the magnetic needle when the latter is at rest. A base-block seven inches long, three inches wide, and seven-eighths of an inch thick is cut and attached to the upright plate by driving screws through the bottom of the plate and into the rear edge of the base. The corners are to be cut from the front of the base, and ten small holes are to be bored half an inch out from the upright and about a quarter of an inch apart. These are for the end wires that will extend down from the coils, and from thence to the binding-post holes. Grooves may be cut in the under side of the base-block for the wires to rest, in, as shown at Fig. 15, which is a view of the inverted base.

A hoop is made of brass, six inches in diameter and an inch wide. It is held to the upright plate with copper wire passed through a small hole, bored at the inner edge of the band, and back through two small holes bored in the plate, the ends being twisted together at the back of the plate. A wire at the top, bottom, and both sides will be sufficient to hold it securely in place.

The first coil of wire is made of No. 18 insulated, and the beginning end is made fast to the binding-post at the left. The wire is carried up through the first hole under the hoop, and after three turns have been made the end is carried down through the second hole and made fast to the foot of the second binding-post.

The second coil is of No. 24 insulated copper wire. The beginning end is made fast to the second binding-post, carried up through the third hole, given five turns about the hoop, drawn down through the fourth hole, and attached to the third binding-post.

The third coil is of the same size wire but has ten turns. The fourth coil has twenty turns, and the fifth, of No. 30 insulated wire, has thirty turns, the last end being attached to the post at the right. In all the coils there should be a total of sixty-eight turns, or about one hundred and five feet of wire.

For strong currents the in-and-out wires may be attached to posts Nos. 1 and 2 at the left, and for weaker currents to Nos. 2 and 3. For still weaker currents, use Nos. 3 and 4, and so on. To detect the very weakest currents, attach the in-and-out wires to the first and last post, and let the current travel through all the coils or the entire length of the wire wound about the hoop.

The magnetic needle is made in the same manner as described for Fig. 10, and the pointer is attached in a similar fashion. But instead of being mounted on a pivot over a card, and so exposed to the open air and possible draughts, the delicate mechanism is arranged within a brass hoop, which is made fast to the ledge. The graduated card is at the bottom of the hoop, or box formed by it, and to protect the needle and prevent it from being displaced it should be covered with glass. This can be done by making a split ring of spring-brass wire and pressing it down inside the hoop. Over this a round piece of glass is placed, and another hoop is pressed in above it to hold the glass in position. If the rings are carefully made and of stout wire they will stay in place; otherwise a drop of melted sealing-wax or paraffine will be necessary to keep them where they are wanted.

The glass should be arranged close enough to the needle to prevent it from jumping or being shaken off the supporting pin, but not so close as to prevent its moving easily.

Part II

Chapter VII
ELECTRICAL RESISTANCE

The science of controlling forces is so well understood and figured out that it becomes a simple mechanical proposition to adapt the various types of controllers to any form of power that may be employed. The tremendous force stored within the mechanism of a great transatlantic liner is governed by the twist of a man’s wrist. The locomotive that will pull a hundred cars loaded with coal, representing a weight of thousands of tons, is started and stopped by a short lever that is drawn in one direction or the other by a man’s hand. Great forces of all kinds are quite as easily controlled as the supply of gas through a jet—by simply turning the key that lets out so much as may be required, no matter what the pressure is back of the flow.

This same principle applies to electricity, but the means of governing it is vastly different from the methods employed for other forces. Electricity is an unknown and unseen force, coming from apparently nowhere and returning to its undiscovered country immediately upon the completion of its work. The flow of steam, water, liquid air, gas, and compressed air through pipes is governed by a throttle or cock, which allows as much or as little to pass as may be required; and if the joints, unions, and couplings in the pipes are not absolutely tight there will be a leakage. Electricity is controlled by resistance in its passage through solid wires, rods, or bars, and cannot be confined within a given space like water, nor held in tanks or pipes as a vapor or gas. It is invisible, colorless, odorless, and occupies no apparent space that can be measured; it is the most powerful and terrible and yet docile force known to man, doing his bidding at all times when properly governed and regulated. In some respects, electricity can be compared to water stored in a tank—for instance, if you have a tank of water containing fifty gallons at an elevation of twenty-five feet, and a pipe leading down from it, the pressure of the water at the outlet of the pipe will be a given number of pounds. Now if the tank were double the size the pressure at the outlet of the pipe would be proportionately greater. Now if you have a battery made up of a number of cells they will develop a given number of volts, and if the number of the cells be doubled the voltage will be correspondingly increased. Or if you have a dynamo giving a certain number of volts, that number may be increased by doubling the size.

The water contained within the tank represents its pressure at the outlet of the pipe. The current in volts, generated in a battery or dynamo, represents its pressure on an outlet or conductor wire; and both represent the force behind their respective conductors. The valve, or faucet, at the end of the pipe plus the friction in the pipe would represent the resistance to the flow of water, while the resistance-coils or other mediums plus the size of the wire, or conductor and switch, would regulate the flow of electric current. The flow of water in a pipe under certain pressure would represent its gallons per minute or hour, while with electricity its flow in a wire or other conductor would represent its amperage. It is to govern the flow of current that resisting mediums are employed.

The resistance of electric current is measured in ohms, and it is with this phase that we are interested in this chapter. If there is only a small resistance put in the path of a current, then it requires but a small pressure or voltage to send it through the wires or circuit. This is easily understood by the boy who has experimented with small incandescent lamps in which short pieces of carbon-filament are contained. It requires the pressure of a few volts only to send the current through the carbon; but for the large carbon-filaments, measuring ten or twelve inches in length, from one hundred to five hundred volts may be necessary. The ordinary house lamps require one hundred and ten volts and half an ampere to give sixteen candle-power.

It is easily understood, then, that it requires a high pressure or voltage to force the current through the resisting carbon-filament, or across the space from one carbon to the other in the arc-lamps used for street lighting. The shorter and larger the conducting wires the less the resistance, and consequently the lower the voltage or pressure necessary to force it. Contrariwise the longer and finer the conducting wares, the greater the resistance. As copper is the best commercial conductor of electric currents, it is in universal use, and in it the minimum of resistance is offered to the current. Iron wire is a poorer conductor, and is not used for high voltage (such as trolleys or transmission of power), but is confined to telegraph and telephone lines and low-pressure work. German-silver wire, one of the poorest conductors, is not used for lines at all, but is employed solely as a resisting medium for controlling current.

Ohm’s Law

This is the fundamental formula expressing the relations between current, electro-motive force, and resistance in an active electric circuit. It may be expressed in several ways with the same result, as follows:

1. The current strength is equal to the E. M. F. (electro-motive force) divided by the resistance.

2. The E. M. F. (electro-motive force) is equal to the current strength multiplied by the resistance.

3. The resistance is equal to the E. M. F. (electro-motive force) divided by the current strength.

All these are different forms of the same statement; and when figuring electrical data, C stands for current, E for electro-motive force, and R for resistance.

Resistance-coils and Rheostats

The method by which electricity is controlled is resistance. No matter how great the voltage of a current, nor its volume in amperes, it can be brought down from the deadly force of the electric trolley-current to the mild degree needed to run a small fan-motor, an electric bell, or a miniature lamp. This is accomplished by means of resisting mediums, such as fluids or wires, which hold back the current, and allow only the small quantity to pass that may be required to operate the apparatus.

The jump from the high voltage of the trolley-current to the low one required for the electric bell, a lamp, or a small motor, is frequently made in traction-work, but in this case regular transformers are used. For the small apparatus, that may have its current supplied from a battery, or a small dynamo driven by a water-motor, the forms of resistance-coils and rheostats described on the following pages should meet every requirement.

The standard unit of resistance is called an ohm, so named after Dr. G. S. Ohm, a German electrician, whose theory on the subject is accepted as the basis on which to calculate all electrical resistance. The legal ohm is the resistance of a mercury column one square millimetre in cross-sectional area and one hundred and six centimetres in length, and at a temperature of 0° Centigrade or 32° Fahrenheit, or the freezing-point for water. The conductivity of metals is dependent greatly on their temperature, a hot wire being a much better conductor than a cold one. Since counter-electro-motive force sometimes gives a spurious resistance, the ohmic resistance is the true standard by which all current is gauged.

In technical mechanism and close readings the ohmic resistance counts for a great deal, but in the simple apparatus that a boy can make the German-silver resistance coils and the liquid resistance will answer every purpose.

To give a clearer idea of the principle of the rheostats, a short description of the mercurial column will first be presented. During the early part of the last century wires were not used as a resisting medium for electric currents. In their place, glass tubes, filled with mercury sealed at one end and corked at the other, were arranged in rows and supported in a wooden rack.

Wires led out from the top and bottom of each tube, and were brought down to metal buttons arranged in a row along the front edge of the base-plate, as shown in the illustration of a mercurial rheostat (Fig. 1). Each tube represented a certain resistance—one or more ohms, as required. The outlet wire was attached to the button at one end of the row, and the inlet could be moved along from button to button, until the required amount of current was obtained.

The mercurial rheostat was an expensive, cumbersome, and treacherous thing to handle; it was liable to break, and its weight often prohibited its use in places where the more convenient and easily handled German-silver rheostats are now in universal employment. Overheating the mercury in the columns caused it to expand, and sometimes, before the switch could be thrown open, an end would be forced out and the mercury would climb over the edge of the glass columns.

All metals have a certain amount of resistance for electric currents, and some have more than others. German-silver, for instance—a metal made of a mixture of other metals with about eighteen per cent. of nickel (see Appendix)—is considered to be the best commercial resistance medium, while pure copper is regarded as the best commercial conductor. Unalloyed copper is universally employed for electric conductors of high voltage; but for telegraph and telephone work, galvanized iron wire is still used extensively.

The finer the wire, the higher is its resistance, and the more resistant the metal, the greater are the number of ohms to a given length. To nine feet and nine inches of No. 30 copper wire there is one ohm resistance, while to No. 24—which is six sizes coarser—there is one ohm to thirty-nine feet and one inch. In many cases it is necessary to use the coarser wire and greater length, as the current would superheat or burn the fine wire, while the coarser would conduct it safely.

For very high voltage and amperage—such as used in traction cars, in power stations, and in manufacturing plants—castings of German-silver are employed and linked in series. They are more easily handled than the coils of wire, and a greater number of them can be accommodated in a small space.

For light currents in experimental work, where batteries are employed, obtain a pound or two of bare German-silver wire, from Nos. 24 to 30, and wind the strands on a round piece of stick attached to a winder (see Magnets and Induction-Coils, chapter iv.). Make several of these coils, two or three inches long, with the wire wound closely and evenly. When pulled apart the coils will appear as shown in Fig. 2 A, and will resemble a spiral spring. This can be made fast over a porcelain knob and the ends caught down, as shown at B in Fig. 2, or it may be drawn over a round stick, a porcelain tube, or a lug made of plaster of Paris and dextrine (three parts of the former to one of the latter), as shown at C in Fig. 2, and the ends securely bound with a strand or two of wire, twisted tight to keep the ends from slipping.

The lugs may be made in a mold, using as a pattern a piece of broom-handle—shellacked and oiled to prevent the plaster from adhering to it. Obtain a small square and deep box, and drop some of the wet mixture down in the bottom; on this place the broomstick, small end down (it should be slightly tapered), and around it pour in the wet plaster mixture. While it is setting, turn the stick with the thumb and fingers, so as to shape the hole perfectly then draw it out, and a true mold will be the result. When dry enough, pour some shellac down into the mold and revolve it, so that the shellac will be evenly distributed, and let it harden for a day. Then saw off the end of the mold, so that it will be open at both ends.

In order to make the lugs, pour in the plaster mixture, taking care to oil the mold before each pouring, so that the lug can be drawn out when the mixture has set. If it sticks, tap the small end gently to start it. For coils where there is little or no heat, ordinary pieces of broom-handle, or round sticks having a coat or two of shellac, will answer very well; but where the current heats the core, it must be of some material that will not char.

Another method of making resistance-coils is to measure off a length of wire; then double it, and with a small staple attach the loop end at one end of the (wooden) core. Pay out the two strands of wire an equal distance apart with the thumb and fingers, and with the other hand twist the core. At the other end of the spool catch the loose ends of the wire under small staples, taking great care not to let the staples touch or even be driven close together. This arrangement is shown at D in Fig. 2, and for a field resistance-board any number of these coils may be made.

In Fig. 3 the mode of connecting coils is shown. The dots represent contact-points to which the movable arm can be shifted. The wires at the bottom of coil, Nos. 1 and 2, are connected together, while those at the top of No. 2 and 3 are joined, and so on to the end. The leading-in current is connected at pole H and so on to J, while the leading-out wire is made fast to pole I. The switch-arm is moved on the first dot, or contact-point, and the current passes up wire A, down coil No. 1, up coil No. 2, down No. 3, up No. 4, and so on to No. 6, and down wire G and out at I. Supposing that this offers too much resistance, the switch-arm is moved up one point. This cuts out coil No. 1, as the current passes up wire B, through coil No. 2, down No. 3, and so on, and out through G and pole I. Another move of the switch and coil No. 2 is cut out, the current passing up wire C, down coil No. 3, up No. 4, and so on, and out at I. Each move of the switch cuts out one coil, lessening the resistance; but when moved to the last contact-point the current flows without resistance—in at H, through the switch-arm, and out at I.

The plan of arranging the coils suggested at Fig. 2 B is shown in Fig. 4, where four of the coils are arranged in series over porcelain knobs, and the lower ends made fast to the base-board with small staples. Small pieces of brass are used for the switch contact-plates; those are provided with one plain and one countersunk hole for a flat and round headed screw.

The screw-heads are arranged in a semicircular fashion, so that the switch-arm, attached at one end to the screw J, will touch each plate as it is moved forward or backward.

A simple arrangement for a resistance-coil is shown in Fig. 5. This consists of a set of small metal plates in which two holes are made, one for a screw, the other for a screw-eye (see Binding-posts, chapter iii.). Two lines of steel-wire nails are driven along a board, and German-silver wire is drawn around them in zig-zag fashion, beginning at the left and going towards the right side of the board. One end of wire is made fast under the screw-head on plate A. The strand is carried out around the first nail on the lower row and over the first one on the upper row, then down, up, down until six nails have been turned. The wire is then carried down to the screw in plate B, given two turns, and carried up again and over the nail on the top row, repeating the direction of zigzag No. 1, until six of them are made. The end of the wire is then made fast to plate G, and all the screws are driven in to hold the plates and wire securely.

The inlet wire is attached to A, the outlet to G, and any degree of resistance can be had by moving the inlet wire to the various plates along the line, cutting out sections Nos. 1 to 6 as desired.

For heavier wire the arrangement as shown in Fig. 6 should be satisfactory.

A frame twelve by fifteen inches is constructed of wood three-quarters of an inch thick and one inch and a quarter wide, having the ends securely fastened with glue and screws. Spirals are wound of German-silver wire (any size from No. 16 to 22), and drawn apart. The ends are caught to the frame with small staples, and each alternate coil-end is joined, as shown in Fig. 6. The leading-out wires to the contact-points on the switch should be of insulated copper, and are to run down the sides of the frame, and so to the switch-board. To clearly illustrate, however, the plan of wiring, the drawing is made so as to show the leads from the coil-ends to the switch. Care should be taken to study this drawing well, so as not to make an error in connecting a wrong end to a contact-point, thereby causing a short circuit. When properly connected the current passes in at A and out at I; but if wires are improperly connected, the current will jump when the switch-arm reaches the misconnected contact.

The switch is an important part of every rheostat, and should be carefully and accurately made. One of the simplest and most practical switches is constructed from a short, flat bar of brass or copper having a knob attached at one end and a hole provided at the other through which a screw may pass (see Switches, chapter iii.). The contact-points are made from one or two copper washers, with the holes countersunk so that a machine screw of brass, with a flat head, will fit the hole snugly. The top of the head will then be flush with the top of the washer, as shown at Fig. 7 A. The bolt is passed down through a piece of board, then slate or soapstone, and caught with a washer and nut, as shown at Fig. 7 B. A loop of wire is passed about the bolt end, then another nut is screwed tightly over it to hold it in place, as well as to lock the first nut. The binding-posts that hold the inlet and outlet wires may be made of bolts and nuts also, as shown at Fig. 7 B; but the bolt must be passed through the switchboard so that the head is at the rear and the ends project out to receive the nuts.

A very compact and simple rheostat and switch is shown in Fig. 8. It is composed of a base-board, eight blocks of hard-wood, and a top strip used as a binder to lock the upper ends of the blocks together. The hard-wood blocks are three-quarters of an inch thick, one inch and a half wide, and four inches long. A small hole is made near each end of the block and through one of them an end of the wire is passed. The wire is then wound round the block, taking care to lay it on evenly, and with about one-eighth of an inch of space between each strand. When the opposite hole is reached, pass the end of the wire through it and clip it. The block will then resemble Fig. 7 C. There should be three or four inches of wire at each end for convenience in connection, and when the eight blocks are wound they are to be mounted on end at the rear side of a base-board measuring ten inches long, three inches wide at the ends, and nine at the middle (or across the face of the switchboard to the rear edge behind the blocks). Use slim steel-wire nails and glue to attach the blocks to the base; or slender screws may be employed. Across the top lay a piece of wood a quarter of an inch in thickness, and drive small nails or screws down through it and into the blocks.

Connect the ends of the coils together in series, as already described, and carry the wires under the base-plate in grooves cut with a V-shaped chisel. If the sunken wires are bothersome, the work may be avoided by running the wires direct to the foot of the contact-points and elevating the rheostat on four small blocks that may be screwed, or nailed and glued, under the corners, as shown in Fig. 8. These will raise the base half an inch or more above the table on which the rheostat will rest so as to allow room for the under wires.

A rheostat of round blocks standing on end is shown at Fig. 9 A. These are pieces of curtain-pole, four inches long and wound with loops of No. 16 or 18 wire, as shown at Fig. 9 B. The loop and loose ends are caught with staples, and when arranged on a base-board they are to be connected in series as before described. One long, slim screw passed up through the base-board and into the lower end of the block will hold each block securely in place. To keep it from twisting, a little glue may be placed under the blocks so that when the screw draws the block down to the base it will stay there permanently upon the hardening of the glue. The leading wires should be slipped under the washers forming the contact-points of the switch; or they may be carried under the board to the nuts that hold the lower ends of the bolts.

Another form of rheostat (Fig. 10 A) is made by sawing a one-inch curtain-pole into four-inch lengths and cross-cutting each piece with eight or ten notches, as shown at Fig. 10 B. These pieces are screwed and glued fast along each side of a base-board eight inches wide and fourteen inches long, so that the notches face the outer edges of the board. The strand of wire passes round these upright blocks and fits into the notches so as to prevent them from falling down.

The top end of wire at each pair of blocks is made fast by a turn or two of another piece of wire and a twist to hold it securely; then the loose end is carried down through a hole and along under the board to the foot of a contact-point.

Any number of these upright coils may be made, and on a long board the switch may be arranged at one side instead of at the end, as shown in Fig. 10 A. When making ten or more coils it is best to use three or four sizes of wire, beginning with fine and ending with coarse. For instance, in a twelve-coil rheostat make three coils of No. 26, three of No. 22, and three of No. 18; or if coarser wire is required use Nos. 20, 16, and 12.

German-silver comes bare and insulated. It is preferable to have the fine wire insulated, but the heavier sizes may be bare, as it is cheaper; moreover, if heated too much the insulation will burn or char off. When cutting out the coils always begin at the end where the finer wire is wound; then as the current is admitted more freely the heavier wires will conduct it without becoming overheated.

For running a sewing-machine, fan, or other small direct-current motor wound for low voltage, the house current (if electric lights are used in the house) may be brought down to the required voltage with German-silver rheostats similar to these already described. Another and very simple method is to arrange sixteen-candle-power lamps in series, as shown in Fig. 11. Six porcelain lamp-sockets are screwed fast to a wood base and the leading in and out wires brought to binding-posts or the contact-points of a switch. The leading-in wire to the series is made fast at binding-post A, which in turn is connected with screw B, under the head of which the switch-arm is held. When the switch is thrown over to contact-point C the current passes through lamp No. 1 back to point D; through lamp No. 2 back to E; then through lamps Nos. 3, 4, 5, and 6, and out through point I to post J. A turn of the switch to D cuts out lamp No. 1, to E cuts out No. 2, and so on. The filaments of incandescent lamps in their vacuum are among the very best mediums of resistance, and with a short series of lamps a current of 220 volts can quickly be cut down to a few volts for light experimental work or to run some small piece of apparatus.

Lamps in series are often used to cut down the current for operating electric toys and trains. The adjustment of the current should never be left to children, however, but should be attended to by some one qualified to look after the apparatus. Otherwise an unpleasant or even dangerous shock may be received. Another simple form of resistance apparatus is made from the carbon pencils used for arc lights. Short pieces will answer very well, but if the long bare ones can be had they will be found preferable. Do not use the copper-plated ones as they would conduct the current too freely; they should be bare and black. Now around the ends of each piece take several turns of copper wire for the terminals and cut-out wires. Fasten those pencils down on a board (as shown at Fig. 12) by boring small holes through the board, passing a loop of copper wire down through the holes, and giving the ends a twist underneath. The leading wires to and from the contact-points should be insulated and may be above or below the board. From the descriptions already given, the connections of this rheostat can readily be understood.

The rheostat shown in Fig. 13 is perhaps the most complete and practical apparatus that a boy could make or would need. It is composed of a frame, six porcelain tubes, a switchboard, and the necessary German-silver and copper wire.

From an electrical supply-house obtain six porcelain tubes fourteen by three-quarter inch. Porcelain tubes and rods warp in the firing and are seldom straight; in purchasing these select them as nearly perfect as possible in shape, size, and length.

Buy, also, twelve small porcelain knobs that are the right size to fit inside the large tubes. These should have holes bored through them to admit screws. Construct a frame of hard-wood to accommodate the tubes, as shown in the drawing, and leave one end loose. With slim screws make the porcelain knobs fast to the top and bottom strips of the frame, as shown in Fig. 14. The porcelain rods will fit over these and will thus be held securely in the frame, one small knob entering the tube at each end, as indicated by the dotted lines in Fig. 14.

The first porcelain tube to the left is wound with No. 22 German-silver wire, the next with No. 20, the third with No. 18, then Nos. 16, 14, and 12; so that in this field a broad range can be had for a current of 110 volts.

The coils are connected in series, as explained for the other rheostats, and the leading wires brought down to the back of a switchboard of which Fig. 13 A is the front and Fig. 13 B the rear view. The switchboard is made of thin slate or soapstone; or a fibre-board may be employed. Fibre-board is especially made for electrical work, and can be had from a large supply-house in pieces of various thickness, three-eighths of an inch being about right for this board. Brass bolts and nuts and copper washers are used for the contact-poles, and when the ends of the leading wires are looped around the bolts the nuts are to be screwed down tightly so as to make good contacts. This rheostat may be used when lying on a table, or it can be hung up by means of two screw-eyes driven in the top of the frame, as shown in Fig. 13 A.

A convenient form of rheostat for fine wire and high resistance is shown in Fig. 15. This is on the plan of the well-known Wheatstone rheostat and does not require a switchboard nor a series of coils. Two rollers, one of wood the other of metal or brass-covered wood, are set in a frame, and by means of a handle and projecting ends with square shoulders, one or the other of the rollers may be turned so that the wire on one winds up while on the other it unwinds.

The wooden roller may be made from a piece of curtain-rod one inch in diameter, and it should have a thread cut on it. This will have to be done on a screw-cutting lathe, and any machinist will do it for a few cents. There should be from twelve to sixteen threads to the inch—no more—although there may be as few as eight. Twelve will be found a good number, as that does not crowd the coils and the risk of their touching is minimized. The ends of the roller should have bearings that will fit in holes made in the end-pieces of the frame, and at one end of each roller a square shoulder is to be cut, as shown at A in Fig. 16. A short handle may be made from two small pieces of wood, as shown at B in Fig. 16. It must be provided with a square hole so that it will fit on the roller ends. The metal roller may be made from a piece of light brass tubing one inch in diameter through which a wooden core is slipped; or it can be a piece of brass-covered curtain-pole with the ends shaped the same as the wooden one. The wood roller should have a collar of thin brass or copper (or other soft metal except lead) attached to the front end; or several turns of wire may be made about the roller so as to form a contact-point. A piece of spring brass, copper, or tin rests on this collar and is held fast under a binding-post, which in turn is screwed to the wooden frame. A similar strip of spring metal is held under another post on the opposite side of the frame and bears on the metal roller.

German-silver wire is wound on the wooden roller, one end having been made fast to the metal collar; and when all the thread grooves on the wood roller are filled the opposite end of the wire is attached to the rear end of the metal roller. The current entering at binding-post No. 1 crosses on the strip of spring metal to the collar, travels along the coil of wire, and crosses to the metal roller and is conducted out at binding-post No. 2 (see Fig. 15). If the resistance is too great slip the handle over the end of the metal roller and give it several turns. The current will then pass with greater freedom as the wire on the wooden roller becomes shorter. This may be readily seen by connecting a small lamp in series with a battery and this rheostat. As the metal cylinder is turned the current flows more freely and the filament becomes red, then white, and finally burns to its full capacity. Take care, however, not to admit too much current as it will burn out the lamp. Some sort of adjustment should be made to prevent the rollers turning of themselves and thus allowing the wire coils to slacken. This may be done by boring the two holes for the rollers to fit in and then, with a key-hole saw, cutting the stick as shown at C in Fig. 16, taking care not to split it at the ends. The result will be a long slot which, however, has nothing to do with the bearings. Down through the middle of the stick make a hole with an awl, so that the screw-eye will move easily in the upper half but will hold in the lower half. Under the head of the eye place a small copper washer; then with the thumb and finger drive the screw-eye down until the head rests on the washer.

A slight turn of the eye when it is in the right place will draw the upper and lower parts of the stick together and bind the wood about the bearing ends of the rollers. The rollers should not be held too tightly as that would strain the wire when winding it from one to the other. It should be just tight enough to keep the wire taut.

Two or more of these roller resistance-frames may be made and connected in series so that a close adjustment can be had when using battery currents for experimenting.

Liquid Resistance

Apart from metallic, mercurial, or carbon resistance a form of liquid apparatus is frequently used in laboratory and light experimental work.