1883.] on Meters for Power and Electricity. 241 



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 indicate 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 angular 

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

 and faster, until centrifugal force overcomes electro-magnetic attrac- 

 tion, 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 difierence in the tightness of the two sides of the 

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

 by the speed ; a power-meter must, therefore, solve this problem — it 

 must subtract the tightness of one side from the tightness of the other 

 side, multiply the difference by the speed at every instant, and add all 

 the products together, continuously representing the growing amount 

 on a dial. I shall now show for the first time an instrument that I 

 have devised, that will do all this in the simplest possible manner. I 

 have here two wheels connected by a driving band of indiarubber, 

 round which I have tied every few inches a piece of white silk ribbon. 

 I shall turn one a little way, and hold the other. The driving force 

 is indicated by a difference of stretching, the pieces of silk are much 

 further apart on the tight side than they are on the loose. I shall 



Fig. 6. 



now turn the handle, and cause the wheels to revolve ; the motion of 

 the band is visible to all. The indiarubber is travelling faster on the 

 tight side than on the loose side, nearly twice as fast ; this must be so, 

 for as there is less material on the tight side than on the loose, there 

 would be a gradual accumulation of the indiarubber round the driven 

 pulley, if they travelled at the same speed ; since there is no accumu- 

 lation, the tight side must travel the fastest. Now it may be shown 

 mathematically that the difference in the speeds is proportional both 

 to the actual speed and to the driving strain ; it is therefore a measure 



R 2 



