METERS FOR POWER AND ELECTRICITY. 371 



it go twice as fast. I will hang on nine pounds. One now goes 

 exactly three times as fast as the other. I will now put four pounds 

 on the first, and leave the nine pounds on the second : the first 

 goes twice while the second goes three times. If instead of a weight 

 we use electro-magnetic force to control the vibrations of a body, 

 then twice the current produces four times the force ; four times the 

 force produces twice the rate ; three times the current produces nine 

 times the force ; nine times the force produces three times the rate, 

 and so on : or the rate is directly proportional to the current strength. 

 There is on the table a working meter made on this principle. I allow 

 the current that passes through to pass also through a galvanometer 

 of special construction, so that you can tell by the position of a spot 

 of light on a scale the strength of the current. At the present time 

 there is no current ; the light is on the zero of the scale, the meter is 

 at rest. I now allow a current to pass from a battery of the new 

 Faure-Sellon-Yolckmar cells which the Storage Company have kindly 

 lent me for this occasion. The light moves through one division on 

 the scale, and the meter has started. I will ask you to observe its rate 

 of vibration. I will now double the current ; this is indicated by the 

 light moving to the end of the second division on the scale : the meter 

 vibrates twice as fast. Now the current is three times as strong, now 

 four times, and so on. You will observe that the position of the spot 

 of light and the rate of vibration always correspond. Every vibra- 

 tion of the meter corresponds to a definite quantity of electricity, and 

 causes a hand on a dial to move on one step. By looking at the dial, 

 we can see how many vibrations there have been, and therefore how 

 much electricity has passed. Just as the vibrating sticks in the model 

 in time come to rest, so the vibrating part of the meter would in time 

 do the same, if it were not kept going by an impulse automatically 

 given to it when required. Also, just as the vibrating sticks can be 

 timed to one another by sliding weights along them, so the vibrating 

 electric meters can be regulated to one another so that all shall indi- 

 cate the same value for the same current, by changing the position or 

 weight of the bobs attached to the vibrating arm. 



The other meter of this class, Dr. Hopkinson's, depends on the 

 fact that centrifugal force is proportional to the square of the an- 

 gular velocity. He therefore allows a little motor to drive a shaft 

 faster and faster, until centrifugal force overcomes electro-magnetic 

 attraction, when the action of the motor ceases. The number of 

 turns of the motor is a measure of the quantity of electricity that has 

 passed. 



I will now pass on to the measurement of power transmitted by 

 belting. The transmission of power by a strap is familiar to every 

 one in a treadle sewing-machine or an ordinary lathe. The driving 

 force depends on the difference in the tightness of the two sides of the 

 belt, and the power transmitted is equal to this difference multiplied 



