Fig. 6. 



372 THE POPULAR SCIENCE MONTHLY. 



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 India-rubber, 

 round which I have tied every few 

 inches a piece of white silk rib- 

 bon. 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 farther apart on the tight 

 side than they are on the loose. I shall now turn the handle, and 

 cause the wheels to revolve ; the motion of the band is visible to 

 all. The India-rubber is traveling 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 India-rubber round the driven pulley, 

 if they traveled at the same speed ; since there is no accumulation, 

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

 matically that the difference in the speeds is proportional both to 

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

 of the power or work being transmitted, and the difference in the dis- 

 tance traveled is a measure of the work done. I have here a working 

 machine which shows directly on a dial the amount of work done ; this 

 I will show in action directly. Instead of India-rubber, elastic steel is 

 used. Since the driving-pulley has the velocity of the tight side, and 

 the driven of the loose side of the belt, the difference in the number of 

 their turns, if they are of equal size, will measure the work. This dif- 

 ference I measure by differential gearing which actuates a hand on a 

 dial. I may turn the handle as fast as I please ; the index does not 

 move, for no work is being done. I may hold the wheel and produce 

 a great driving-strain ; again the index remains at rest, for no work is 

 being done. I now turn the handle quickly, and lightly touch the 

 driven wheel with my finger. The resistance, small though it is, has 

 to be overcome ; a minute amount of work is being done, the index 

 creeps around gently. I will now put more pressure on my finger, 

 more work is being done, the index is moving faster ; whether I in- 

 crease the speed or the resistance, the index turns faster ; its rate of 

 motion measures the power, and the distance it has moved, or the num- 

 ber of turns, measures the work done. That this is so I will show by 

 an experiment. I will wind up in front of a scale a seven-pound weight ; 

 the hand has turned one third around ; I will now wind a twenty-eight- 

 pound weight up the same height ; the hand has turned four thirds of 



