Fig. 623.--New moving element of Keystone instruments, weight 1.2 grams.
Fig. 624.--Moving element of Keystone instruments assembled in bearing. The moving element consists of coil, counterpoise and pointer. The mechanical connections are made by means of screws and steady pins. In order to adjust for slight set or subset of spring under long use a zero adjuster is provided by means of which this set can be connected and the pointer brought back to zero.
Ques. Why is a high resistance coil used with a voltmeter?
Ans. As actually constructed, most voltmeters are simply special forms of ammeter. From Ohm's law, the current through a given circuit equals the pressure at its terminals divided by its resistance. Hence, if a high resistance be connected in series with a sensitive ammeter that will measure very small currents, then the current passing through the circuit is directly proportional to the voltage at its terminals, and the instrument may be calibrated to read volts.
Figs. 625 and 626.--Connections for series and shunt ammeters. When the construction is such that all the current passes through the instrument, it is connected as in [fig. 625], but where the instrument is designed to take only a fraction of the current, it is connected across a shunt, as in [fig. 626], a definite proportion of the current passing through the instrument and the remainder through the shunt.
Ques. Into what two classes may ammeters be divided?
Ans. They are classed as series or shunt according to the way they are designed to be connected with the circuit.
Ques. What determines the mode of connecting ammeters?