WE have next to consider the appliances for creating the necessary charging electromotive force, and for storing and releasing this charge at pleasure, so as to generate the required electrical oscillations in the aerial.
It is essential that this generator should be able to create not only large potential difference, but also a certain minimum electric current. Accordingly, we-are limited at the present moment to one of two appliances, viz., the induction coil or the alternating current transformer.
It will not be necessary to enter into an explanation of the action of the induction coil. The coil generally employed for wireless telegraphy is technically known as a ten-inch coil, i. e., a coil which is capable of giving a ten-inch spark between pointed conductors in air at ordinary pressure. The construction of a large coil of this description is a matter requiring great technical skill, and is not to be attempted without considerable previous experience in the manufacture of smaller coils. The secondary circuit of a ten-inch coil is formed of double silk-covered copper wire, generally speaking the gauge called No. 36, or else No. 34 S.W.G. is used, and a length of ten to seventeen miles of wire is employed on the secondary circuit, according to the gauge of wire selected. For the precautions necessary in constructing the secondary coil, practical manuals must be consulted.
- Instruction for the manufacture of large induction coils may be obtained from a 'Treatise on the Construction of Large Induction Coils,' by A. T. Hare. (Methuen & Co., London.)