The heating effect of currents is also inde pendent of the direction of the current flow, so that hot wire instruments may be used for measuring alternating currents and voltages.
When very large currents are to be meas ured, it is often inconvenient and expensive to build an ammeter with conductors large enough to carry the entire current. In these instances an ammeter of small capacity may be shunted by a German silver wire or rod and the shunted instrument may then be calibrated and used to measure the large current. This arrange ment has become quite universal in the large electric light works where very great currents are to be measured and it is not uncommon in ordinary portable instruments. Indeed, nearly all Weston self-contained ammeters, such as illustrated in Fig. 3, consist of a milli-ammeter arranged with a proper shunt E inside the case.
An entirely distinct method of measuring voltage is by means of electrometers, and when these are converted into portable form for everyday use they are called electrostatic volt meters. They are particularly useful for meas suring alternating voltages.
The electric power which is used in any part of a continuous current circuit may be determined by measuring by means of an am meter the current flowing through the circuit and measuring by means of a voltmeter the voltage at the terminals of the circuit. In a direct current circuit the product of the num ber of amperes by the number of volts gives the power in watts. This product called the instantaneous values of the product must be secured. Instruments are made which in them selves perform this double measurement and multiplication so that their indications are directly proportional to power and these in struments are called wattmeters. The simplest form is an electrodynamometer in which one coil is wound with many turns of fine wire exactly as though it were to be used as a volt meter coiliand the other coil is wound with a few turns of coarse wire as though it were to be used in an ammeter.
The action of such a wattmeter is best ex plained by an illustration. Suppose it is de sired to measure the power used by an electric .motor— the fine wire coil of the wattmeter is connected across the terminals of the motor and the coarse wire coil of the wattmeter is connected in series with the motor. The magnetic effect of the fine wire coil is then proportional to the voltage at the motor ter minals and the magnetic effect of the coarse wire coil is proportional to the current flowing through the motor. The force exerted at any instant, to move the movable coil, is propor tional to the product at that instant of the two magnetic effects and the pointer moves over the scale so as to indicate the average force, thereby indicating the watts which are trans mitted through the circuit.
The connection of a wattmeter in a circuit is diagrammatically illustrated in Fig. 10, where W is the wattmeter, V V' are the terminals of the fine wire coil and C C' are the terminals of the coarse wire coil. This figure is intended to show a dynamo furnishing current to a set of incandescent lamps LLLL L, and the watt meter is introduced in circuit for the purpose of measuring the power delivered to the lamps.
The number of alterations made in each second by the alternating currents that are ordinarily used in practice is so great that the movable coil of an electrodynamometer acts exactly as though it were pulled around by a continuous force proportional to the average of the variable force which results from the magnetic action of the alternating current This is true whether the instrument is arranged to be used as an alternating-current ammeter, in which instance the two coils are placed in series with each other and both are of low resistance; or the instrument is arranged to be used as a voltmeter, in which instance the two coils are connected in series with each other but both are of relatively high resistance and their resistance is re-enforced by that of a supplementary non-inductive coil of high re sistance; or the instrument is arranged for tise as a wattmeter, in which instance one of the coils is of low resistance and is a current coil to be connected in series with the main circuit, while the other coil is a fine wire coil of higb resistance re-enforced by a supplementary non inductive resistance coil and is a voltage coil to be connected across the terminals of the circuit The wattmeter already described is called an indicating wattmeter, but it is often desirable to make a registry of the integrated consump tion of energy by a customer during a given period, such as a month, and such an instrument is illustrated in Fig. 11, which is called the Thomson watthour meter. This is built like a little electric motor without any iron in its work ing parts. It is arranged with its revolving part or armature A. to be connected to the circuit like the fine wire coils of a wattmeter and its field magnetizing windings W W to be connected in circuit like the coarse wire coils of a watt meter. The magnetic pull which tends at each instant to make the armature rotate is propor-. tional to the product of the two magnetizing effects, so that the rotative effect, or torque, is proportional to the watts in the circuit. If the speed of such an armature is made propor tional to the magnetic pull, it is easily seen that every revolution of the armature means a cer tain number of watts used for a fixed length of time. Such instruments usually have at When the power in alternating current cir cuits is to be measured, it is necessary to make. the self-induction of the fine wire coil of such a wattmeter practically negligible in compari son with the resistance of the coil, to prevent the readings of the instrument being affected tacked to the spindle of the armature a set of dials D like those of a gas meter, which record the revolutions and are so marked that the consumption of electric energy may be re corded in what are known as ((watt hours." A more convenient unit for commercial instru ments is the kilowatt hour, each kilowatt hour being 1,000 watt hours, and the dials of such instruments usually read in kilowatt hours.