Voltmeters (Fig. 236), or instruments for measuring voltage, are like ammeters except that a wire of very high resistance is in circuit with the movable coil. In external appearance they are not distinguishable from ammeters.
FIG. 236.—A voltmeter.
The unit of electromotive force is called the volt. The voltage of a dry cell is approximately 1.5 volts, and the voltage of a voltaic cell and of a gravity cell is approximately 1 volt.
316. Current, Voltage, Resistance. We learned in Section 287 that the strength of a current increases when the electromotive force increases, and diminishes when the electromotive force diminishes. Later, in Section 288, we learned that the strength of the current decreases as the resistance in circuit increases.
The strength of a steady current depends upon these two factors only, the electromotive force which causes it and the resistance which it has to overcome.
317. Resistance. Since resistance plays so important a rôle in electricity, it becomes necessary to have a unit of resistance. The practical unit of resistance is called an ohm, and some idea of the value of an ohm can be obtained if we remember that a 300-foot length of common iron telegraph wire has a resistance of 1 ohm. An approximate ohm for rough work in the laboratory may be made by winding 9 feet 5 inches of number 30 copper wire on a spool or arranging it in any other convenient form.
In Section 299 we learned that substances differ very greatly in the resistance which they offer to electricity, and so it will not surprise us to learn that while it takes 300 feet of iron telegraph wire to give 1 ohm of resistance, it takes but 39 feet of number 24 copper wire, and but 2.2 feet of number 24 German silver wire, to give the same resistance.
NOTE. The number of a wire indicates its diameter; number 30, for example, being always of a definite fixed diameter, no matter what the material of the wire.
If we wish to avoid loss of current by heating, we use a wire of low resistance; while if we wish to transform electricity into heat, as in the electric stove, we choose wire of high resistance, as German silver wire.