Ques. How does the winding differ in ammeters and voltmeters? Ans. An ammeter coil consists of a few turns of heavy wire (when designed to carry the full current), while a voltmeter coil is wound with many turns of fine wire. Thus, the ammeter is of low resistance, and the voltmeter of high resistance. [Illustration: 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. [Illustration: 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? Ans. When the wire of the ammeter coil is large enough to carry the whole current, it is connected in the circuit _in series_ as shown in fig. 625. If, however, the wire be small, the instrument is connected _in parallel_ with a shunt of low resistance, so that it only carries a small part of the current, as in fig. 626. For circuits which carry large currents, the shunt connection is always used, because otherwise the coil of the ammeter would have to be very heavy and the instrument correspondingly bulky. Ques. How are shunt ammeters arranged to correctly measure the current?