Fig. 62.
Voltmeters are instruments by means of which E. M. F. can be read on a printed scale. They are a variety of galvanometer, and are made with coils of such high resistance, compared with the resistance of a cell or dynamo, that the E. M. F. can be read direct. The reason for this will be seen by referring to Ohm's law ("Study," § 356); the resistance is so great that the strength of the current depends entirely upon the E. M. F.
Fig. 63.
Voltmeters measure electrical pressure just as steam gauges measure the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63 shows a voltmeter with illuminated dial. An electrical bulb behind the instrument furnishes light so that the readings can be easily taken.
68. Electrical Resistance. Did you ever ride down hill on a hand-sled? How easily the sled glides over the snow! What happens, though, when you strike a bare place, or a place where some evil-minded person has sprinkled ashes? Does the sled pass easily over bare ground or ashes? Snow offers very little resistance to the sled, while ashes offer a great resistance.
Fig. 64.
All substances do not allow the electric current to pass through them with the same ease. Even the liquid in a cell tends to hold the current back and offers internal resistance. The various wires and instruments connected to a cell offer external resistance. (See "Study," Chapter XVIII., for experiments, etc.)