The scale may be completed by marking the positions of the pointer
when other currents of known strength flow through the ammeter.
[Illustration: FIG. 235.--Marking the scale of an ammeter.]
All electric plants, whether for heating, lighting, or for machinery,
are provided with ammeters, such instruments being as important to an
electric plant as the steam gauge is to the boiler.
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.
|