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. [Illustration: 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. [Illustration: Fig. 629.--Voltmeter connection for measuring the pressure in an electric circuit. The voltmeter is connected in parallel in the circuit at the point where the voltage is to be measured. Fig. 630.--Voltmeter connection for measuring the "drop" or fall in voltage in a certain length of wire, as for instance, the length between the points A and B. The voltmeter is shunted between the two points whose pressure difference is to be measured.] Ques. What is the difference between a voltmeter and an ammeter? Ans. A voltmeter measures pressure, while an ammeter measures current. As actually constructed, most voltmeters are simply special forms of ammeter. 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 pressure or voltage at its terminals and the instrument may be calibrated to read volts. Ques. Explain the term "calibrate."