In winding primary coils it is proposed to reduce the self-induction or inductance of its adjacent coils by means of similar methods used in winding electromagnets. The primary winding, instead of being composed of a number of turns of one large wire, is made up of a multiple winding of small wires, aggregating the conductivity of the large wire. This materially reduces sparking at the contact breaker, and certainly allows of a closer bedding of wire nearer the core, also giving a greater percentage of ampere turns. Another scheme which uses the Dessauer contact breaker provides two separate primary windings, opening one when the other closes. Such schemes as these come well within the scope of the experimenter, and it is highly possible that valuable improvements will be made in coil design during the coming years.

Failure to Work.

The following are the commonest causes of coils not working to their best limit: Contact breaker contacts dirty, burned, stuck, too small, not in good parallel relation face to face of platinum.

Secondary wires crossed outside coil, often happens that the secondary is quietly sparking away into or through some object touching it, particularly when long wire connections are run from secondary to place of desired sparking.

Condenser too small, burned out, badly insulated (see other pages on this subject).

Battery too small—too high internal resistance or wires leading from battery to coil too small—for ordinary coil work, distance of, perhaps, ten feet, use No. 10 to 12 B & S flexible lamp cord or solid wire. Ruhmkorff coils require plenty of current to produce large sparks.

Dimensions for Different Spark Lengths.
½ inch1 inch2 inches6 inches12 inches
Foil sheets5½ × 46 × 46 × 610 × 512 × 8
Number4040606060
Paper sheets6½ × 59 × 58½ × 712 × 714 × 10
Number6060808080
Core length5791219
Core diameter⅝¾1″1⅛1½
Primary size B & S1614141210
Secondary size B & S.3636363638
Core wire size B W G.2222222222
Quantity in pounds
of secondary wire		¾1¼2½712
Layers of primary33222
Area of paper, sq. in.2,0002,7004,8006,60011,000
Area of foil, sq. in.8809602,1003,0005,760

As it is not always convenient to procure paper and foil in set sizes, the area of material needed for condensers is also given. The above table is approximate. It represents data collected from the best modern practice. The gauge above given for copper wire is that of Brown & Sharpe, and is used throughout these pages.

Medical Coils.

The main points of difference between coils for electrotherapeutics and Ruhmkorff coils is that the former are devoid of condensers, are rarely insulated to a high degree, and are arranged for current strength regulation. The modes of regulation are many, briefly the principal are: (a) In coils with independent circuit breakers, sliding both core and primary coil out of the secondary together or independently. (b) Moving a metal tube over or off the primary coil or core or both. Many combinations of these methods are practised. Attempts have been made to regulate battery current by rheostat, but it is not feasible, except in large stationary outfits. Cheap medical coils are wound with bare wire, with layers of thread between adjacent turns, or even only bedding the wire turns in paraffined paper. It is not intended to convey the idea that winding bare wire coils is a makeshift; far from it. This method is being very generally adopted in telephone work. But it requires special and delicate machinery, and is unsuited to amateur work, where slight differences of cost or labor are insignificant. Others for specific purposes consist of a primary coil only. The best and most complete made are so arranged that independent secondary coils of different sized wires can be used with the one primary, being readily slipped on or off as required. There is another scheme of regulation, where the coil is wound in sections and these sections cut in or out by means of a switch, but it is not desirable.