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?
Ans. The coil is arranged so that a definite proportion of the whole current passes through it. A large conductor of low resistance is connected directly between the two terminals or binding posts of the instrument; the coil is connected as a shunt around a definite part of this main conductor; then, since the two are connected in parallel and each branch has a definite resistance, the current divides between the two branches directly in proportion to their relative conductivities, or inversely according to their resistances. The coil, therefore, takes a definite part of the whole current, and the force moving it and its pointer away from the zero position is directly proportional to the whole current. Hence, by providing a proper scale, the value of the entire current will be indicated.
Figs. 627 and 628.--Westinghouse ammeter shunts. These shunts are used where heavy currents are to be measured. The shunt is connected in series with the bus bar or circuit to be measured, and its terminals are connected by means of small leads to the ammeter or other instrument. These shunts are designed to have approximately 50 millivolts drop at full rated current. They are intended primarily for Westinghouse meters, but can be used satisfactorily with any meter requiring 50 millivolts for full scale deflection.
Ques. How is a voltmeter connected?
Ans. A voltmeter is always connected to the two points, whose difference of potential is to be measured.
For instance, to measure the voltage between the two sides A and B of the circuit shown in [fig. 629], one terminal of the voltmeter is connected to wire A, and the other to wire B. If the "drop" or difference in voltage through a certain length of wire L, of a circuit, as from A to B in [fig. 630] is to be determined, one terminal of the voltmeter is connected to A and the other to B. In a similar manner is found the drop through a lamp.
