Disruptive "Tesla" Coil.
For Fig. 11 the specification is as follows: Secondary, 300 turns of No. 30 B & S silk-covered magnet wire, wound on rubber tube or rod, and the ends encased in glass or rubber tubes. This is inserted into the primary, which consists of two coils, each of 20 turns No. 16 B & S rubber-covered wire, wound separately on a long rubber tube not less than ⅛ inch thick. The last tube must be large enough to be very loose when the secondary coil is inserted in it, and it must project at least two inches over each end of the secondary. A hard rubber division must be placed between these primary coils. The four ends of the latter coils are connected C C to two condensers and D D to two discharger balls, the secondary wires going to the exhibitive apparatus. A further description of these connections is to be found in [Chapter XII]., also notes upon the use of the disruptive coil.
Coils for Gas Engines.
These are either primary only or primary and secondary. Two to three pounds of No. 14 B & S magnet wire are wound on an iron wire core eight to ten inches in length by one inch in diameter. The contact is made and broken in the igniter of the engine as at the wipe spring of a ratchet gas burner. Four to eight large cells of dry battery are used, or eight cells Edison-Lalande—iron-clad type. Number of cells varies with size of coil needed, some classes of engines require a heavier spark than others to ignite the vapor.
When a primary and secondary are used, the primary should be made of two or three layers No. 14 B & S magnet wire, and a secondary of one pound No. 34 B & S magnet wire. There can be an independent contact breaker or the coil can be made up similar to a one-half inch spark Ruhmkorff coil (see Chapter I.).
Fig. 12.
The method of connecting up a coil of the latter description is shown in Fig. 12, which is self-explanatory. It shows a form of cam-shaft switch which is operated by the engine, and which opens and closes the primary circuit of the induction coil, the sparks from the secondary winding passing between the points of the igniter in the engine cylinder. As shown in Fig. 12, the igniter or ignition plug is similar in operation to a coil discharger, the two terminals being, however, insulated from each other by the use of porcelain. To ensure a good insulation under the severe working conditions has been somewhat of a task, but it seems to have been attained in the types of igniters known as the Splitdorf and the Roche or New Standard.
The Splitdorf gas-engine coil is the result of much experiment and careful design. It is built to stand hard usage, and the insulation used has been adopted only after exhaustive test. In automobile work, where a heavy strain is made upon the engine, as in climbing heavy grades, it has been found that a stronger spark gives surer results. This would indicate more battery current through the coil, and it is a wise precaution to have a few extra cells attached that can be switched on if necessary.
In constructing spark coils for gas engines particular care must be given to the contact breaker. In most types of gas or oil vapor engines it is absolutely necessary to have the spark pass with uniform regularity, and immediately and surely when required. For automobiles or where the apparatus is subject to jar, a heavy iron vibrating armature would become unreliable by reason of its inertia and its responding to shock. At every jolt of the vehicle it would jar and get out of rhythm, and it certainly seems preferable to use a mechanical contact apparatus whenever feasible. In the older type of gas engine the spark is made by mechanism breaking contact right in the vapor. The actual arrangement of these devices is detailed and illustrated in the later works on gas and oil engines.