Fig. 568.--Ratio coils of Wheatstone bridge. Almost every box intended to serve as a Wheatstone bridge is furnished with a set of coils which forms the arms of proportion or ratio arms of the bridge. There is a choice of several different ways of arranging these coils. The figure shows the simplest arrangement, which is employed in boxes not intended for the highest accuracy. The required ratio, as for instance 1:100, is obtained by withdrawing a plug from each arm A and B. Ratios 1/1, 1/10, 1/100, 10/100, etc., or 1,000/1, 1,000/10, 1,000/100, 100/100, etc., are obtainable in this manner. This simple arrangement is open not only to the objection that the contact resistance of the plugs which remain in is always included with the resistance unplugged, but also to all other objections to be urged against the use of many plugs where a few will do. The method has the limitation that it is not possible to reverse the arms of the bridge, that is, to transpose the arms A and B.
Ques. Why should the battery key be depressed before the galvanometer key?
Ans. To avoid the sudden swing of the galvanometer needle, which occurs on closing circuit in consequence of self-induction.
Ques. How is it known whether too much or too little resistance be unplugged?
Ans. The galvanometer needle will be deflected to one side for too much resistance, and to the opposite side for too little resistance.
Fig. 569.--Method of reversing arms of Wheatstone bridge with reversing blocks. The arrangement shown in the figure is classical, being that used in the English post office type of Wheatstone bridge. It is open to the objections which apply to the use of several plugs, one of which is withdrawn to obtain the desired resistance.
Ques. What is the meaning of "Inf.," marked on the bridge?
Ans. It stands for "infinity," because the resistance coil at the point marked infinity is omitted so that adjacent sections of the arm are disconnected when the plug is taken out.
In fact, the air gap interposed by the removal of the plug by no means provides an infinitely great resistance, but is usually called such because it is vastly greater than any of the other resistances of the bridge.
Figs. 570 and 571.--Diagrams illustrating the decade plan of combining resistance coils. In this method the coils are connected in series and the arrangement avoids the disadvantage of the ordinary Wheatstone bridge in that the latter requires a large number of plugs to short circuit the resistances not in use, which introduces an element of uncertainty as to resistance of the plug contacts and the necessity of adding up the values of all the unplugged resistances in order to determine the total resistance in circuit. The necessary regular succession of values in a rheostat built on the decade plan can be obtained with either nine or ten coils per decade. The chief reason for using the latter number is found in the facility with which all the coils of one decade can be compared with one coil of the next higher decade, thus permitting the coils of a rheostat to be checked among themselves. Thus, the ten 1 ohm coils can be checked with a 10 ohm, the ten 10's with a 100, etc. In some sets the ten coils of a decade can be connected in series or in parallel, and it then becomes an advantage to have ten coils to a decade, since the coils in one decade in parallel equal one of the coils of the next lower decade. When these latter advantages are not required, and especially when dials or sliding switches are used, there is little or no advantage in using more than nine coils per decade, as shown in fig. 570. Here all the coils of the set are connected in series so that the circuit is never open. Thus it is a slight advantage to have permanent connections a, b, and c, because all the coils of a decade can be thrown in circuit by simply pulling out a plug, it not being necessary to insert it again, as would be the case if the a, b, and c connections were not used. Moreover, if any plug make bad contact, its effect is somewhat lessened by having this bad contact shunted by the remaining coils of the decade. Again, there are occasions where violent deflections of a galvanometer are prevented by not having the circuit entirely open when a plug is taken out.
The Decade Plan.--In this method of combining resistance coils, there are 9 or 10 one ohm coils for the units place, 9 or 10 ten ohm coils for the tens place, 9 or 10 one hundred ohm coils for the hundreds place and so on. Each series of coils of the same value is designated a decade. The connections are usually made as shown in figs. 570 and 571.
It is apparent from the figure that any value in any one decade can be obtained by inserting between a bar and a block, only one plug; moreover if several decades be in series, any value up to the limit of the set can be read off directly from the position of the plugs without having to add up the unplugged resistance as in the ordinary arrangement.
Fig. 572.--Two plug arrangement of ratio coils. Each of the ratio coils has one of its terminals connected to a common center which corresponds to the block marked C in the figure. The other terminal of each coil is connected to an individual block, there being one block for each coil. The bar B on one side of these blocks is joined to the rheostat and the bar A on the other side to an X post. In the ordinary use of this set of ratio coils two plugs only are used. One plug is inserted between the bar A and one of the blocks, 1, 1', 10, 10', etc., of the central row of blocks. The other plug is inserted between the bar B and any one of the other blocks of the central row. There are two ratio coils of each value. To obtain an even ratio as 1,000 to 1,000', one plug is inserted between the block 1,000 and the bar A, and the other plug between the 1,000 block and bar B, the ratio arms are reversed; that is, the 1,000 ohm coil is connected to the X post and the 1,000 ohm coil to the end of the rheostat. When uneven ratios are used, the same ratio can be obtained by four different combinations. To obtain the ratio one to ten, insert a plug between A and 1, and another between B and 10, or between A and 1', and B and 10, and get 1:10, or between A and 1, and B and 10', and get 1:10', or again, between A and 1' and B and 10', and get 1' to 10'. Other ratios are obtained in a similar manner. By using more than two plugs and connecting certain of the coils in parallel combinations, a large number of other ratios may be obtained. This arrangement offers a convenient method of measuring the sensibility of a bridge and galvanometer combination that is frequently applicable. If for instance the one ohm coil is used on either side after a balance has been obtained the one ohm may be shunted with the 1,000 ohm on the same side. This will make a variation of 1/10 of 1% and the galvanometer deflection may be noted for this variation. Similarly, the 1 ohm may be shunted with the 100 for a variation of 1%, or with the 10,000 for a variation of 1/100 of 1%. The ten ohm coil may be shunted with the 1,000 for a variation of 1% and with the 10,000 for a variation of 1/10 of 1%. In the arrangement of ratio coils, errors due to plug contacts are negligible because only two plug contacts enter the circuit, and with an even ratio, it is only the difference in the resistances of the two plug contacts that can affect the result. In measuring any of the ratio coils while in the box it is only necessary to connect to the bar C and to either the bar A or B and plug in the coils to be measured.
Figs. 573 and 574.--The Leeds and Northrup decade. The object of this arrangement is to reduce the number of coils required. In fig. 573, the 1, 3', 3 and 2 are connected in series. Let the terminals of the 1 ohm and 2 ohm coils be numbered (1), (2), (3), (4) and (5) (fig. 573). The current enters at point (1) and leaves the coils at the point (5), traversing 1, 3', 3, 2 = 9 ohms in all. If this series be multiplied by any factor n, then n (1 + 3' + 3 + 2) = n 9 ohms. It will be seen that if the points (1) and (5) be connected, all the coils are short circuited, and the current will traverse zero resistance. If the points (2) and (5) be connected, the 3', 3 and 2 ohm coils will be short circuited and the current will traverse 1 ohm. By extending the process so as to connect two and only two points at a time it is possible to obtain the regular succession of values n (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the last being obtained when no points are connected. Fig. 574 shows Leeds and Northrup's method of connecting these points two at a time with the use of a single plug. The circles in the diagram represent two rows of ten brass blocks each. To the first two blocks at the top of the rows, the points (5) and (1), fig. 573, are connected; to the second two, the points (2) and (5) are connected, etc., no points being connected to the last pair of blocks. Hence, if a plug be inserted between blocks 1 and 5, fig. 575, the points (1) and (5) of diagram fig. 573 are connected, giving the value of 0, if between the blocks 2 and 5 the points (2) and (5) are connected, giving the value 1, and so on. The value 9 is obtainable when the plug is in the last pair of blocks, which have no connections. Fig. 572 shows a top view of the blocks of a simple decade constructed upon this plan.
Ques. What other advantages are gained with the decade arrangement?
Ans. The single plug used with each decade is never out of use, being either in the zero position or set on some value, and is therefore not easily lost by being laid aside. The use of only one plug in a decade makes it easy to ascertain that the plug is making good contact as only one block in a row is plugged at a time, the other blocks are not kept under a strain by having plugs forced tightly between them.
This strain on the blocks, which always exists in those sets in which a resistance is thrown in by removing a plug, tends to separate or loosen them and often to warp the hard rubber upon which they are mounted. Another advantage of the decade plan is that it permits obtaining a succession of values by means of sliding contacts or dial switches, a method which is becoming deservedly more appreciated.
Fig. 575.--Leeds and Northrup dial Wheatstone bridge. Rotating switches are used instead of plugs, which permits quicker adjustment of the resistances, adapting it to rapid working. The ratio coils are arranged as in fig. 568. There are four dials which form the rheostat. The units dial contains 9 one ohm coils; the tens dial, 9 ten ohm coils; the hundreds dial, 9 one hundred ohm coils, and the thousands dial 9 one thousand ohm coils. The values of the ratio coils are 1, 1, 10, 10, 100, 100, 1,000, 1,000, 10,000, 10,000.
Ques. What is the difference between "plug out" and "plug in" types of resistance box?
Ans. In the plug out type, resistance is put in the circuit by removing plugs, as in fig. 565; in the plug in type, resistance is put in the circuit by inserting plugs as in figs. 570 and 571.
Fig. 576.--Queen Acme portable testing set. It consists of a Wheatstone bridge, with reversible arms, battery of four dry cells, D'Arsonval galvanometer, battery and galvanometer keys. There are sixteen resistance coils, having a combined resistance of 11,110 ohms. Each bridge arm is provided with three coils of 1, 10, 100 ohms, and 10, 100, 1,000 ohms respectively. The commutator admits of a ratio of 1 to 1,000 on either bridge arm, giving the set a theoretical range from .001 of an ohm to 11,110,000 ohms. For resistances above 1,000,000 ohms, the normal battery power must be increased. The contact keys are located as shown. The battery key has single contact, but the galvanometer key has double contact; depressing it closes the galvanometer circuit, and releasing it short circuits the galvanometer, bringing the latter quickly to rest.
Testing Sets.--For convenience in testing, a combination of the instruments used is put up in a neat and substantial case, and known as a testing set. There are innumerable forms of testing set, a few of which are shown in the accompanying illustrations. The usual combination is a Wheatstone bridge, galvanometer, battery and necessary keys and connections.
Fig. 577.--Connections and circuits of Queen acme portable testing set. There are three rows of blocks, LL', MM', NN'. LL is connected to NN' by means of a heavy copper bar, joining L' and N'. LL' and NN' constitute the rheostat, from which any resistance from 1 ohm to 11,110 ohms may be obtained by removing the proper plugs. The block N of the rheostat is connected to the lower line post D. The upper line post C is connected to the block X of the commutator. The block C has no other permanent connection, except key G. The block R of the commutator is connected to the block L of the rheostat, and has no other connection, excepting by plugs. Each half of MM' constitutes a bridge arm, designated A and B respectively. Beginning at the lower line post D, the connections form a continuous circuit through the rheostat, thence through the bridge arm B, thence through the bridge arm A, thence to the upper line post C. The commutator serves merely to reverse the bridge arms A and B. The battery terminals are connected as shown: the positive terminal directly to the common junction of the two bridge arms, and the negative terminal through the battery key to the rheostat. The positive terminal of the galvanometer is connected through the galvanometer key with the block X, and the negative terminal with the block R of the commutator or what is equivalent, with the block L of the rheostat. The commutator blocks A, B, R and X, are connected by plugs as shown. When the commutator plugs are in the position PQ, the bridge arm B is connected to the rheostat and the bridge arm A is connected to the line, the ratio between the bridge arms ratio being A ÷ B = X ÷ R but when the plugs are in the position ST, the bridge arms are reversed in position A, being connected with the rheostat and B, with the line, and the bridge arm ratio becomes A ÷ B = R ÷ X. The connections of the testing set may be more readily understood from the simplified diagram fig. 578.
Fig. 578.--Simplified diagram showing connections of Queen Acme portable testing set.
Ques. Describe the operation of the Queen Acme testing set figs. 576 and 577, in measuring resistance.
Ans. Connect the terminals of the resistance to be measured to the line posts C and D. Place the battery connections on the two upper tips 0 and 1, thus throwing one end of the battery into circuit, which is sufficient until an approximate balance is obtained. Employ the 100 ohm coil in each bridge arm, and place the commutator plugs in the position PQ, or in the position ST. Then remove plugs from the rheostat until the value of total resistance employed, or nearly as may be guessed is equal to that of the unknown resistance. Now press the battery key Ba, and holding it down momentarily, press the galvanometer key Ga. If the galvanometer needle swing to the right toward the symbol + the resistance employed in the rheostat is too high and must be reduced. If the needle swing to the left toward -, the resistance employed is too low and must be increased. By altering the resistance of the rheostat accordingly, a value will soon be found, which when varied slightly either way, will reverse the deflection of the galvanometer needle. Now remove the battery connection from tip 1, and place it on the tip 4, thus throwing the whole battery into circuit. Then press the keys again as before, first the battery key, then the galvanometer key. This will increase the deflection of the galvanometer needle for the same variation in the rheostat, thus enabling the making of a more accurate adjustment. The measurement thus made will be the best result that can be obtained with bridge arms of equal value, but by selecting more suitable values of the two arms from the following table of bridge ratios a much higher degree of accuracy may be obtained.
Fig. 579.--Diagram of the Queen dial decade portable testing set. Its dimensions are 9-1/2" long, 7" wide and 7" deep, and weighs 11-1/2 pounds. The resistances are arranged upon the dial decade plan, being placed in circuit by means of a rotating switch contact. The switches are so constructed that they may be turned in either direction, thereby permitting them to be turned quickly from the highest resistance in any dial to the lowest resistance in the same dial. This arrangement avoids the necessity of turning back through all the remaining resistances in any particular group of coils and is of value in locating swinging crosses or conditions of momentary balances. The connections for the various tests are made by the manipulation of one small knife switch (W.B.--M.L.) and the switch BA.; these are plainly lettered, thus avoiding the necessity of referring to a diagram of connections. In construction, the dial switches are made up of eight laminations of No. 28 B. & S. phosphor bronze, and the form is such as to prevent wearing grooves on the top of the contact studs. In this instrument the electrical circuits are soldered throughout excepting the switch contact whose resistance is negligible. The resistances are wound with manganin. The battery comprises six cells sub-divided which are easily replaceable. The galvanometer is the same as in the Queen acme set, but has the addition of an Ayrton shunt, which is useful in making insulation measurements. The necessary keys, binding posts, and switches are provided so as to facilitate the use of the instrument for the various measurements that can be made with it.
Table Showing the Best Values of Bridge Arms for Measuring any Desired Resistance
| Value of Resistance being measured | Best values of | Position of Commutator Plugs as shown in fig. 582 |
|
| A = | B = | ||
| Below 1.5 ohms | 1 | 1,000 | PQ |
| Between 1.5 and 11 ohms | 1 | 100 | PQ |
| " 11 and 78 ohms | 10 | 100 | PQ |
| " 78 and 1,100 ohms | 100 | 1,000 | PQ |
| " 1,100 and 6,100 ohms | 100 | 100 | PQ or ST |
| " 6,100 and 110,000 ohms | 1,000 | 100 | ST |
| " 110,000 and 1,110,000 ohms | 1,000 | 10 | ST |
| " 1,110,000 and 11,110,000 ohms | 1,000 | 1 | ST |
Ques. In testing with the Queen Acme set how should the plugs be placed in the commutator?
Ans. Always make the arm A the smaller except when the two arms are of equal value.
Ques. If the resistance being measured is higher than 6,100 ohms, or lower than 1,100 ohms, how should the commutator plugs be placed?
Ans. If higher than 6,100 ohms, they should be placed in the position ST; if lower than 1,100 ohms, in position PQ.
When the plugs are placed in the ST position, the unknown resistance is found by dividing the value of the larger bridge arm by that of the smaller, and multiplying the total employed resistance in the rheostat by the quotient. When the plugs are placed in the PQ position, the employed resistance in the rheostat is divided by the quotient.
Fig. 580.--Queen portable silver chloride testing battery. The silver chloride cell has the advantage of long life, light weight, and compactness. The pressure of each cell when new is .8 volt.
Direct Deflection Method with Queen Acme Set.--To measure for instance, insulation resistance by direct deflection connect a known high resistance, say 100,000 ohms between the line post C (fig. 577), and the positive battery post. Remove all plugs from the commutator, and place all plugs in the rheostat, as any employed resistance in the rheostat will be in circuit with the galvanometer and the battery. Place the battery connection so as to throw only one cell into circuit. Now press the keys and obtain a deflection of the galvanometer needle. For example: assume that the needle to be deflected about 8 divisions of the scale. Since this deflection is due to the current from one cell passing through a resistance of 100,000 ohms, then 100,000 × 8 = .8 megohms represents the resistance through which one cell will produce a deflection of one division on the scale. Hence, .8 megohms is the constant of the galvanometer.
Fig. 581.--Ohmmeter. It consists essentially of a slide wire Wheatstone bridge, with the scale divided to read either directly in ohms, or in per cent. of a fixed resistance value. A galvanometer is mounted on the containing case of each, and battery and galvanometer keys are provided. In the direct reading type, the scale is so cut that when the galvanometer is balanced, the pointer of the instrument indicates the value of the resistance between the X posts. The scale is calibrated for any desired range. These ohmmeters being slide wire bridges, the greatest accuracy is at the center of the scale, hence one should be selected that will bring the part of the scale likely to be the most used at or near the center. A convenient type is that in which the scale is cut in per cent., 100 per cent. being at the center of the scale. Fixed coils of 1, 10, 100, 1,000 and 10,000 ohms are contained in the instrument with a plugging arrangement allowing any one to be used. When a balance is obtained, the actual resistance is determined by multiplying the dial reading by the value of the fixed coil in use. This amounts simply to shifting the decimal point. For instance, if the 100 ohm coil were being used, and the pointer were at .875, the resistance would be 87.5 ohms.
Now, replace the known high resistance (100,000 ohms) by the unknown resistance (for instance such as a cable) the value of which is to be determined. Add enough cells to produce as large a deflection of the needle as possible. Assume that 75 cells give a deflection of 1.5 scale division. Then, the galvanometer constant multiplied by the number of cells and the product divided by the deflection will give the insulation resistance of the cable; or
as the resistance of the cable.
Fig. 582.--Commutator plug setting for comparing electromotive forces by the fall of potential method with Queen acme set.
Fall of Potential Method with Queen Acme Set.--To compare electromotive forces by this method, place the battery connection (fig. 577), so as to throw into circuit all the cells, taking care not to reverse them by crossing the battery cords. Plug the commutator as shown in fig. 582, and remove 1,000 ohms from bridge arm B. Place all plugs in arm A. From the rheostat unplug 5,000 ohms. Then connect one of the cells being tested, with its positive terminal to the + battery post and its negative terminal to the line post C.
When the keys are pressed, the galvanometer needle will swing either to the right or to the left. If it swing toward +, reduce the resistance in the rheostat; if it swing toward -, add resistance to the rheostat. When a value is found wherein a variation of an ohm either way reverses the deflection, add to this value the resistance unplugged in arm B, and divide the sum by the resistance in arm B. The result gives the ratio between the voltages of the testing set battery and cell being tested respectively. The division is decimal and may be readily accomplished by merely pointing off as many places as there are ciphers in the resistance employed from arm B. This operation repeated with any number of different cells, will give their voltages in terms of the voltage of the testing set battery, and from these ratios their relative values may be readily obtained.
Fig. 583.--Diagram of apparatus for measuring low resistances based on the principle of the Kelvin double bridge. In the diagram AB represents a heavy piece of resistance metal of uniform cross section and uniform resistance per unit of length; CD is another piece of resistance metal of smaller cross section, and the two are joined together by a heavy copper bar, AC, into which both are silver soldered; LL are the current terminals and PP are the pressure terminals. The resistance of AB between the marks 0 and 100 on the scale S is .001 ohm. From the point 1 on the resistance CD to 0 on AB is also .001 ohm, from 2 to 0 is .002 and so on, and from 9 to 100 is .01 ohm. The slider M moves along the resistance AB and its position is read on the scale S which is divided into 100 equal parts and can be read by a vernier to thousandths. Subdivided in this way the resistance between the tap off points PP may have any value from .001 to .01 ohms by steps of .000001 ohm.
If the testing set battery be replaced by a standard cell, the first measurement gives at once the voltage of the cell tested.
If the voltage of the cell or battery being tested exceed that of the testing set battery, reverse the position of the two batteries, and the subsequent operations, as outlined above, will give the desired results.
How to check a Voltmeter with the Queen Acme Set.--In using a set as in fig. 576, first remove about 10,000 ohms from the rheostat, plug the commutator as shown in fig. 582, remove 100 ohms from the arm B, of the bridge, and connect a standard cell with the positive terminal to the + battery post and the negative terminal to the line post C. Then, connect the circuit to the battery posts of the testing set the positive lead to the + post and the negative lead to the - post. Now, press both keys and note the direction of the deflection of the galvanometer needle. If it move toward +, the rheostat resistance is too high; if toward -, too low.
Fig. 584.--Kelvin bridge. This includes a low resistance standard of .1 ohm variable by steps of .00001 ohm, a set of ratio coils, and a holder for rods or wires to be measured, with a scale to measure their length. It is also provided with heavy flexibles to be used in measuring the resistances of irregularly shaped pieces. The connections are clearly shown in the diagram. The range of measurements of this bridge is: 1 ohm to .1 ohm by steps of .001 ohm readily estimated to .0001; .1 to .01 ohm by steps of .0001 ohm readily estimated to .00001; .01 ohm to .001 ohm by steps of .00001 ohm, readily estimated to .000001; .001 ohm down by steps of .00001 ohm, readily estimated to .000001 ohm.
Change the rheostat resistance accordingly until the balance attained is such that a very slight variation of the rheostat resistance one way or the other will reverse the galvanometer deflection. To find the pressure on the circuit, add 100 to rheostat resistance and point off two places. Multiply this value by the voltage and the product will be the desired voltage.
If the voltage of the standard cell be exactly one volt, the total employed resistance represents the voltage on the circuit.
Fig. 585.--Queen slide wire bridge. It consists of a portable slide wire, Wheatstone bridge arranged to read directly in ohms in addition to its use for locating crosses and grounds. It is complete with battery, galvanometer and telephone receiver. The bridge is balanced by moving the hand stylus until the galvanometer shows no deflection or until there is no sound in the telephone receiver. In order to provide a wide range of measurement and maximum accuracy, ratio coils or multipliers having values of 1, 10, 100, 1,000 and 10,000 are provided. The scale of the instrument is arranged in two parts, one of which indicates ohms and the other is divided into uniform divisions for use when locating crosses and grounds by the Murray and Varley loop methods. A small induction coil is included so as to furnish an alternating current when using the telephone receiver.
For instance, in making a measurement on a 110 volt circuit, assume that the employing of 7,840 ohms rheostat resistance produces balance, and that increasing or decreasing this resistance by two ohms, reverses the galvanometer deflection. This indication that the setting 7,840 is uncertain, about 1/40 of 1 per cent. Since the rheostat coils are adjusted to an accuracy of only 1/5 of 1 per cent., that will be about the accuracy of the measurement.
If the pressure of the standard cell be 1.018 volts, then 7,840 + 100 = 7,940. Pointing off two places, gives 79.40, which multiplied by 1.018 gives 80.82 for the voltage on the circuit.
To Measure Internal Resistance of Cell with Queen Acme Set.--First compare its voltage on open circuit with the pressure of the testing set battery. Then, shunt the cell with a known resistance, about 100 ohms, and again measure its terminal voltage. The difference between the two values thus obtained, divided by the value of the shunt resistance, will give the value of the current. To find the internal resistance, multiply the value of the shunt resistance by the ratio between the first and second measured values.
Fig. 586.--Evershed portable ohmmeter set. This testing set consists of a direct reading ohmmeter which indicates by direct reading the value of the resistance being tested, also a portable hand dynamo which provides at any required pressure the current necessary to make the test. It is adapted to the needs of supply stations, wiring contractors and dynamo builders. It is also useful in testing the insulation of underground and aerial cables, and is designed so that it can be used by ordinary workmen who are not experienced in handling delicate instruments and who, by its use, are able to obtain accurate results. The dynamo is wound for 100, 200, 500, or 1,000 volts, and is fitted with spring drum inside the case on which is coiled a twin flexible cord provided with a connector adapted for clamping under the ohmmeter terminals.
For instance, assume that the open circuit voltage of the cell being tested as compared with the voltage of the testing set battery is .212 of the latter, and that when it is shunted with a resistance of 1,000 ohms, its terminal voltage is .179. Then, the total resistance is to the 1,000 ohms shunt resistance as .212 is to .179 or (.212/.179) × 1,000 = 1,184, from which deducting the 1,000 ohms shunt resistance, gives 184 ohms as the internal resistance of the cell.
Fig. 587.--Leeds and Northrup fault finder. A lineman's instrument for the location of faults, crosses, grounds, and opens in telephone and telegraph circuits, and for the measurement of conductor and insulation resistance.
Fig. 588.--Diagram showing arrangement and connections of Leeds and Northrup fault finder. It is used to measure conductor resistance, fault resistance, to locate faults by four different tests, and when used with a buzzer and telephone, to locate opens. The essential feature of the instrument is the uniform resistance AB, which lies in a circle and which has a value of about 100 ohms. By a special construction, it is so arranged that the contact can be made at any point along it, and it is therefore equivalent to a very high resistance slide wire. It has a moving contact C and a uniform scale of 1,000 divisions. In series with this, there are the two resistances E and R which may be short circuited by the switches U and V. E has exactly the same resistance as the wire AB. R has a resistance of 100 ohms, and is the fixed resistance of the bridge arrangement for resistance measurements. The resistances of 1,000 ohms and 9,000 ohms connected to the battery post are to protect the battery and the apparatus from excessive current. The 9,000 ohms may be short circuited by the switch W.
Ammeter Test with Queen Acme Set.--Connect a low resistance in series with the ammeter and run leads from it to the testing set, the positive lead to the + battery post and the negative lead to the line post C (fig. 577). Insert a standard cell between the battery posts, with positive terminal to + battery post, and negative terminal to - battery post. Plug commutator as shown in fig. 582. Remove 10,000 ohms from rheostat, and 100 ohms from bridge arm B. Determine a balance in the usual way by changing the value of the resistance in the rheostat. This operation will balance the difference of pressure at the terminals of the shunt resistance against the standard cell, and its value is equal to
To determine the current flowing, divide the value of the difference of pressure thus obtained by the value of the shunt resistance.
Fig. 589.--Resistance measurement with Leeds and Northrup fault finder. The diagram shows the proper connections and switch settings for measuring conductor resistance. As in the ordinary slide wire bridge, the resistance X between the two posts 1 and 2 is obtained from the formula X = A ÷ (1,000 - A) × R. To avoid the necessity of solving in each case the fraction A ÷ (1,000 - A), a table is furnished with the instrument, giving the value of this fraction for each value of A. The resistance is accordingly determined in each case by simply setting the contact C for a balance and reading from the table the resistance opposite the number corresponding to the scale reading and multiplying by 100, the value of R. To use an outside battery, remove the inside battery and connect the outside battery between the posts Gr and Ba. The pressure of this battery should not exceed 110 volts. If it exceed 25 volts, open switch W.
EXAMPLE--With an unknown resistance connected between the posts 1 and 2, the galvanometer showed a balance for a dial reading of 387. The number opposite 387 in the table is .6313; hence, X = .6313 × 100 = 63.13 ohms.
Fig. 590.--Diagram of the Queen standard potentiometer. The circuit arrangement is a method of sub-dividing the main potentiometer wire, MNOPQ, so as to provide for very accurate reading. The secondary voltage, or that used to supply current to the main potentiometer circuit, is adjusted by regulating rheostats, "Fast," "Medium," and "Slow" so that the current flow is exactly .0001 ampere. It is noted that this instrument requires a very small current for its operation. The instrument is direct reading for voltage measurements, not exceeding 1.4+. In order to determine if the current flow through the potentiometer be exactly .0001 ampere, the terminals of the standard cell binding posts are connected in circuit so that the drop over points between which they are connected are exactly equal to the voltage of the standard cell used. Binding posts are provided for connection with various standard cells. The unknown voltage to be measured is placed in opposition to the current flow in the potentiometer circuit by connecting to the binding post "XEMF." Observe that polarity is connected as required. The galvanometer with its shunt is placed in the standard cell circuit, or X circuit, by means of a double pole, double throw switch. The switch at T provides for standard cells of different values and the setting at U allows for temperature correction. The range of the instrument in volts can be increased by means of multipliers or volt boxes.