Nikola Tesla Books
HIGH FREQUENCY APPARATUS To simplify the first formula into a form where it is ready for use at a moment's notice without any calculation, we may say that the number of cycles per revolution will be equal to the number of poles divided by 2. To use this, let us take the case of the four pole machine. Four divided by 2 is 2. Therefore, the machine will deliver two cycles to every complete revolution of the rotor. If the speed is 1800 revolutions per minute or 30 per second, the frequency is 2 times 30 or 60 cycles per second.* Effects of Change of Frequency.-For commercial use such as lighting lamps and operating motors, the 60-cycle current is in general use in the United States. Certain parts of the country still use 125 and 133-cycle currents and in Canada the 25-cycle current is much in evidence. A change in the frequency of the current necessitates profound changes in the apparatus it is intended to operate. It is not within the province of this work, however, to touch upon the alterations necessary in motors in order that they may be adapted for various frequencies. Suffice it to say that in the case of transformers, which are closely identified with the apparatus described, any change in the frequency of the current necessitates a corresponding change in the windings of the transformer. Generally speaking, one of the higher frequencies is to be preferred for transformer work, for the core may be lighter and smaller, and the instrument is consequently cheaper and easier to build. Therefore, if the worker intends to generate his own alternating current, he may well employ an alternator producing a 120-cycle current at moderate speed. As a rule, however, some form of alternating current supply is available and, in such event, the experimenter will, of course, find it cheaper and better to *For a comprehensive treatment of the theory of alternating currents, see "Electricity at High Potentials and Frequencies."--Transtrom, $2.00.
