Induction Coils, How to Make, Use, and Repair Them. / Including Ruhmkorff, Tesla, and Medical Coils, Roentgen Radiography, Etc. Etc.

Figure 14 shows a coil with tube mode of regulation. The core C consists of a piece of iron tube, very thin, 4 inches long by ⅜ inch diameter, and filled with soft iron wires. One end of this core is firmly fixed in the left-hand bobbin head. The object of the iron tube is to prevent the sliding tube from catching in the iron wires, otherwise it can be dispensed with. Over this tube is slipped a brass tube T, ending in a handle H at the right-hand end; this must work easily over the core tube. The spool for the primary is now made up by fixing the other bobbin head on a paper or fibre tube and fastening its free end to the left-hand bobbin head, or the spool can be made in the usual way by glueing up two spool ends on a fibre or paper tube and securing the iron core firmly in one end, allowing room, of course, for the brass tube to slide in at the right-hand end. The primary winding is three or four layers of No. 20 B & S gauge cotton-covered magnet wire, the ends being brought out for future connection. Over this is now laid a few layers of paraffined paper, and ten or twelve layers of No. 36 B & S cotton-covered magnet wire is wound on for the secondary coil.

The contact breaker R is in no way different from the simple form described in Chapter II. Its construction can be readily seen from the figure.

A layer of cloth of the kind used in covering electromagnets is laid on over the secondary, and the coil is ready to be attached to the base. The base is seven inches long by three wide, and has little feet at its four corners to elevate it from the table and prevent abrasion of the connections underneath.

The connections are as given in Fig. 15. When in operation, the electrode cords being attached to binding posts, Nos. 1 and 2 are in circuit with the secondary coil only. When at Nos. 2 and 3 they receive the induced current or extra current in the primary, caused by the break of the battery circuit (see page 3).

Medical Coil with Interchangeable Secondaries.

This form of coil is the only one for practical medical work, and more space will be given to its construction than to the foregoing, which is suited only for limited use.

Fig. 16 shows side elevation of coil on base. The design can be largely varied, also it can be used either for a wall board, a cabinet top, or made to be carried in a case containing battery, electrodes, etc. S is one of the secondary coils, of which at least three should be provided. The dimensions are, of course, the same—namely, four inches long by 3½ inches wide over all. The spool ends are furnished with heel pieces, which slide under the brass track bar T. This accurately centres the coil and prevents it from working loose.

Windings for Secondary.

The following windings for removable or interchangeable secondary coils are those most in use.

Coil No. 1. 4500 feet (.375 pound) No. 36 B & S, approximating 1800 ohms. This may be led out in three divisions by means of switch on coil head. First division, 4500 feet; second division, 3000 feet; third division, 1500 feet.

Coil No. 2. 2400 feet (.6 pound) No. 31 B & S, about 350 ohms, divided into 2400 feet, 1500 feet, and 900 feet.

Coil No. 3. 750 feet (1 pound) No. 22 B & S in one coil, or two divisions of 500 and 750 feet, respectively; approximate resistance of wire, 125 ohms.

Coil No. 4. It may be necessary to obtain currents of extremely high tension, in which case a coil may be prepared of 5000 feet No. 38 B & S, or No. 40 B & S preferably.

The finer the wire, the less current and the most sedative effect; the coarser the wire, the more current with corresponding increased painful action.

The spools, in fact as much of the framework as possible, should be made of hard rubber, to which a fine finish can be given, although mahogany, rosewood, or even stained oak can be used. On each side of the right-hand spool heads a flat brass spring is screwed, making the contact for the secondary wires on brass strips screwed on top of the track rods. These secondary connections can be made by means of flexible cords to binding posts, but the sliding contact is preferable. The primary coil P is firmly held in the left spool head, and consists of a core of No. 22 B W G soft iron wires, insulated and wound with three layers of No. 20 B & S magnet wire. The outside of this coil is neatly enclosed in a hard rubber tube to permit of the secondary coils sliding freely upon it. It is better, however, for the secondary coils not to touch the primary tube. The vibrator, or contact breaker, should be of the adjustable form shown in Fig. 17. The adjusting screw for the contact breaker can be mounted in a brass lug carried by the spool head.

Connections of this coil are substantially the same as those of the first-described medical coil. This apparatus is well worthy of elaboration; it should be fitted with a ribbon vibrator as well as an adjustable speed slow vibrator, a switch controlling either. A great variety of secondary coils can be made, those of coarse wire taking the place of the current from the contact breaker. The vibrators should be operated from an independent battery, although in the last coil described the magnet may be wound with the same size wire as the primary and then be in series with it. The secondary spools can be made of stained hard wood ends fitted on to fibre tube, which latter is easily procurable. Particular attention should always be paid to the spools and heads; if not properly made, they may come apart, and a disastrous unravelling of the wires ensues.

Bath Coils.

A coil much used for electric baths has a primary winding only, regulated by the sliding in and out of the iron core, which necessitates the use of an independent vibrator, or else by varying the current strength with a rheostat. The general directions given before will answer in the present case, the only data necessary being the size of wire, which should be about six to ten layers of No. 20 B & S. The coil with movable secondaries here comes into service. Strong currents are needed for bath work, and any variety of winding can be used with this make of coil. There are so many descriptions of bath and small medical coils in the electrical magazines published for amateur workers, that it is hardly necessary here to give more than a mention of the principal ones.

Hints in Caring for Medical Coils.

A few remarks on medical coils and their diseases may not be amiss; often a very little defect, if remedied in time, will prevent costly repairs.

The main care in medical electrical apparatus is the battery (see Chapter X. for descriptions of coil batteries and their operation). Clean, fresh solutions and clean contacts are essential. Keep zincs well amalgamated, remove wires from binding posts, and scrape bright the metal where the wires make connection; see no fluid is splashed on contacts, clean all contact springs periodically. The Edison-Lalande battery is probably the best for medical use, but even this requires occasional attention as to contacts, new zincs, fresh solution, etc.

Poor adjustment at contact breaker, dirty or corroded contacts, loose wires, loose binding posts, corroded binding posts, are often the only trouble in a coil refusing to work.

Flexible cords are fruitful of trouble: the tinsel breaks, and there is no circuit; gets wet and crosses or causes a leak; cord tips get loose and alternately open and close a contact; one minute all is well, next minute no current can be obtained. Another trouble in acid batteries is caused by leaving the zincs in the fluid. It is easy to do it in most cases, although the ingenuity of the leading medical electrical apparatus makers to-day is directed to this point. Cleanliness and careful inspection of all contacts is well repaid; carelessness surely brings its evils.

It is very desirable in medical work to eliminate the noise attendant upon the working of the coil vibrator. This jarring or humming is often in itself a source of irritation to a nervous patient. The sound can be deadened in various ways, for instance, by placing over the vibrator a temporary wood cover, lined with felt, resting upon a soft rubber gasket; or in any other manner that may suggest itself to the operator.

Table Showing Resistances and Feet Per Pound of Copper and German Silver Wires.