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.
Resistance Coils.
Although foreign to the title of this book, these coils will be mentioned, being often necessary as accessories to the operation of coils, wireless telegraphy, etc. These are coils of insulated German silver wire, wound to a specified resistance. The main feature about those designed for testing is that they are wound non-inductively—that is, the wire is wound double in such manner that the current flows both ways around the turns, and so neutralizes the inductive action. In cases where dynamo current is to be used, as in telegraphs operated from dynamo current, the coils are wound on tin tubes to make them fireproof and yet radiate the heat. As the resistance of German silver varies very largely, only approximate figures can be given. The table ([page 64]) has been made up from the best averages obtainable. The carrying capacity of resistance coils varies with their construction, the better they can radiate heat, the more current they can safely carry.
General Remarks on Coils, etc.
Ruhmkorff induction coils should always be fitted with a switch to open, close, or reverse the power circuit, a double throw, double pole, baby knife switch, mounted on a separate porcelain base, is very suitable. Such a switch is open when the handle is vertical, and it should always be left so when changing connections, fixing battery, etc. A large, well-finished coil will have the secondary wires brought in rubber tubes to binding posts mounted on hard rubber pillars, or to binding posts mounted considerably above the coil cover level. A very neat mode is shown in the frontispiece on the large 45-inch spark coil. Here the secondary wires go to hard rubber pillars, which also carry adjustable rod dischargers. These rods are movable towards or away from each other by means of the large hard rubber handle to which they are connected by a simple system of levers. In this coil the secondary is moulded on a flexible tube, which fits loosely over the primary tube in order to compensate for changes of temperature and consequent expansions and contractions. All well-designed coils should be so arranged that the primary coil and core can be readily removed from the secondary, or vice versa. It is sometimes desirable to use a different primary. This arrangement will greatly facilitate any necessary repairs. It must be always remembered that the working of a coil depends on the insulation between primary and secondary. Spare no pains to have perfect insulation; it is a hopeless task to reinsulate a broken-down secondary, although the sectional method of winding facilitates repairs. In large winding rooms it is customary to have a revolution counter connected to the spindle, so that the number of turns can be seen at all times. A bicycle cyclometer can be readily fitted up for this purpose, and will be found of considerable assistance where a number of sections are needed, each with a similar number of turns. In the commercial construction of telephone coils and magnet spools it is often the rule to specify only the number of turns of the requisite size wire, the ampere turns of the coils being thus regulated.
The Testing of a Coil for Polarity.
This is often necessary, and may be done in a variety of ways. When the coil is working, and sparks be passed between fine wires mounted on the discharger, the positive wire tip will be cold, whereas the negative end will be quite hot. In vacuo, the positive shows a purple red when the negative glows with a bluish violet. The decomposition of water, which consists of oxygen and hydrogen in the formula H2O, is readily accomplished by the secondary current, and the greatest volume of gas (hydrogen) will be evolved at the negative pole. For ready reference a summary of these facts is given below:
| Positive | Negative |
|---|---|
| Cold wire, | Hot wire, |
| Anode, | Cathode, |
| + sign, | - sign, |
| Purple red, | Bluish violet, |
| Zinc plate, | Carbon plate, |
| (Carbon) pole, | Zinc pole, |
| Oxygen gas. | Hydrogen gas. |