In presenting this book on the Tesla coil to the public the authors hope that they have filled a long felt vacancy in the practical library of science. No attempt has been made to give a mathematical explanation of the oscillation transformer, and other parts of the high-frequency apparatus, for the simple reason that the theory is too complex, and when obtained of no practical use. Neither have the authors tried to lead the amateur, who is just learning how to string bells and connect batteries, from the elements of the galvanic cell up to the working of a high-potential, alternating current, but have merely made an effort to place in the hands of advanced amateurs in electrical science a practical working manual on the construction of high-frequency coils, now so useful in scientific investigation. The attention of the authors was first called to the Tesla coil when they were fortunate enough to be given the use of the 7" standard coil described in the last chapter of this book. A systematic line of experiments was carried on with it, in order to study the effects of a change in the constants of the various circuits.