Resistance is opposition to the movement of the current. If it is offered by the electrolyte, it is designated "Internal Resistance." If, on the other hand, the opposition takes place, for instance, through the wire, it is then called "External Resistance."
The electrolyte must be either acid, or alkaline, or saline, and the electrodes must be of dissimilar metals, so the electrolyte will attack one of them.
The current is measured in amperes, and the force with which it is caused to flow is measured in volts. In practice the word "current" is used to designate ampere flow; and electromotive force, or E. M. F., is used instead of voltage.
Electro-magnetic Electricity.—The third method of generating electricity is by electro-magnets. The value and use of induction will now be seen, and you will be enabled to utilize the lesson concerning magnetic action referred to in the previous chapter.
Magnetic Radiation.—You will remember that every piece of metal which is within the path of an electric current has a space all about its surface from end to end which is electrified. This electrified field extends out a certain distance from the metal, and is supposed to maintain a movement around it. If, now, another piece of metal is brought within range of this electric or magnetic[p. 38] zone and moved across it, so as to cut through this field, a current will be generated thereby, or rather added to the current already exerted, so that if we start with a feeble current, it can be increased by rapidly "cutting the lines of force," as it is called.
Different Kinds of Dynamo.—While there are many kinds of dynamo, they all, without exception, are constructed in accordance with this principle. There are also many varieties of current. For instance, a dynamo may be made to produce a high voltage and a low amperage; another with high amperage and low voltage; another which gives a direct current for lighting, heating, power, and electroplating; still another which generates an alternating current for high tension power, or transmission, arc-lighting, etc., all of which will be explained hereafter.
In this place, however, a full description of a direct-current dynamo will explain the principle involved in all dynamos—that to generate a current of electricity makes it necessary for us to move a field of force, like an armature, rapidly and continuously through another field of force, like a magnetic field.
Direct-Current Dynamo.—We shall now make the simplest form of dynamo, using for this purpose a pair of permanent magnets