315.  Voltage and Voltmeters.  Since electromotive force, or voltage, is the cause of current, it should be possible to compare different electromotive forces by comparing the currents which they produce in a given circuit.  But two voltages of equal value do not give equal currents unless the resistances met by the currents are equal.  For example, the simple voltaic cell and the gravity cell have approximately equal voltages, but the current produced by the voltaic cell is stronger than that produced by the gravity cell.  This is because the current meets more resistance within the gravity cell than within the voltaic cell.  Every cell, no matter what its nature, offers resistance to the flow of electricity through it and is said to have internal resistance.  If we are determining the voltages of various cells by a comparison of the respective currents produced, the result will be true only on condition that the resistances in the various circuits are equal.  If a very large external resistance of fine wire is placed in circuit with a gravity cell, the total resistance of the circuit (made up of the relatively small resistance in the cell and the larger resistance in the rest of the circuit) will differ but little from that of another circuit in which the gravity cell is replaced by a voltaic cell, or any other type of cell.

With a high resistance in the outside circuit, the deflections of the ammeter will be small, but such as they are, they will fairly accurately represent the electromotive forces which produce them.

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.

[Illustration:  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 role 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.