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Updated: June 20, 2025
The unit of electromotive force is the volt, and this is the electric pressure which will force a current of 1 ampere through a resistance of 1 ohm; it is expressed by the symbol E. A fresh dry cell will deliver a current of about 1.5 volts. Resistance and the Ohm.
A definite relation exists between the current and its electromotive force and also between the current, electromotive force and the resistance of the circuit; and if you will get this relationship clearly in your mind you will have a very good insight into how direct and alternating currents act.
I usually set the pieces by putting between them sheets of uniform thickness at a certain very small distance which is known from the experiments of Sir William Thomson to require a certain electromotive force to be bridged by the spark. It should, of course, be remembered that the sparking distance is much diminished as the frequency is increased.
The committee appointed to test experimentally Ohm's law, that with any conductor the electromotive force is proportioned to the current produced, reports that this law is absolutely correct. This fine result is the more gratifying since Ohm's law is entirely empirical and does not rest at all upon logical deduction.
Kapp's experiment was not conclusive, for the length of spark is not an accurate measure of electromotive force. As regards Mr. Mordey's observation, he thought the action explicable on the theory of the leading condenser current acting on the field magnets.
A definite relation exists between the current and its electromotive force and also between the current, electromotive force and the resistance of the circuit; and if you will get this relationship clearly in your mind you will have a very good insight into how direct and alternating currents act.
High electromotive forces or "pressures" capable of overcoming high resistances outside the battery can be obtained in this way. This arrangement does not increase the electromotive force, but diminishes the resistance. In fact, the battery is equivalent to a single cell having plates equal in area to the total area of all the plates.
Voltaic cells are grouped together in the mode of Leyden jars. This arrangement multiplies alike the electromotive force and the resistance. The electromotive force of the battery is the sum of the electromotive forces of all the cells, and the resistance of the battery is the sum of the resistances of all the cells.
The field-magnet circuits of the dynamos were connected in multiple with a separate energizing source; and the field-current; and strength of field, were regulated to maintain the required amount of electromotive force upon the supply-mains under all conditions of load from the maximum to the minimum number of lamps in use, and to keep the electromotive force of all machines alike."
But before he has long experimented with the extreme frequencies required to set up in a small bulb an electromotive force of several thousands of volts he realizes the great importance of electrostatic effects, and these effects grow relatively to the electro-dynamic in significance as the frequency is increased.
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