Fig. 655.--Recording dials of watt hour meter, illustrating method of reading electric meters. The unit of measurement of electrical energy is the watt hour. 1,000 watt hours make or equal 1 kilowatt hour. Some electric meters have 4 dials, the extreme right hand dial of which registers in kilowatt hours, while others have 5 dials, the extreme right hand dial of which registers in tenths of kilowatt hours. In making out bills to customers the extreme right hand dial of a 5 dial meter is ignored in order that the "state of meter" shown on bills uniformly requires the addition of 3 ciphers to correctly express the registration in watt hours. Each division on the right hand dial (ignoring the 5th dial mentioned) denotes 1,000 watt hours or 1 kilowatt hour; on the next dial 10 kilowatt hours, on the next dial 100 kilowatt hours and on the left hand dial 1,000 kilowatt hours. One complete revolution of any dial causes the hand on the dial immediately to its left to move forward one division. To take a statement from the meter begin at the left and set down for each dial the lower figure next to each hand, not necessarily the figure nearer the hand. In the above example the statement is 1,726 kilowatt hours or 1,726,000 watt hours. Subtract the previous statement to arrive at registration for a given period. Some meters are subject to a multiplying constant so stated on their face and the registration of such meters must be multiplied by the constant as shown, to determine the actual consumption of electrical energy. The constant is the measure of the mechanical adjustment in the register of the meter and is the ratio between the registration of the dial hands and the true consumption. This adjustment is made always by the manufacturer of the meter and is never changed in service.] Ques. What is the action of the motor in the Thompson watt hour meter? Ans. It rotates at very slow speed, and since there is no iron in its fields and armature, it has very little reverse voltage. Its armature current, therefore, is independent of the speed of rotation, and is constant for any definite voltage applied at its terminals.