238 Mr. C. Vernon Boys [March 2, 



board to incline a disc in contact with a roller, and then drag the 

 roller steadily along by clockwork. The number of turns of the 

 roller would give the quantity of water. Instruments that will thus 

 add up continuously indications at a time, and so find amounts during 

 a time, are called integrators. 



The most important application that I have made at present of 

 the integrator described, is what I have called an engine-power meter. 

 The instrument is on the table, but as it is far too small to be seen 

 at a distance, I have arranged a large model to illustrate its action. 



The object of this machine is to measure how much 

 Fig. 4. work an engine has done during any time, and 



show the result on a dial, so that a workman may 

 read it off at once without having to make any 

 calculations. 



Before I can explain how work is measured, per- 

 haps I had better say a few words about the 

 meaning of the word "work." Work is done when 

 pressure overcomes resistance, producing motion. 

 Neither motion nor pressure alone is work. The 

 tw^o factors, pressure and motion, must occur to- 

 gether. The work done is found by multiplying 

 the pressure by the distance moved. In an engine, 

 steam pushes the piston first one w^ay then the other, overcomes resist- 

 ance, and does work. To find this, we must multiply the pressure by 

 the motion at every instant and add all the products together. This 

 is what the engine-power meter does, and it shows the continuously 

 growing result on a dial. When the piston moves it drags the 

 cylinder along, where the steam presses the wheel is inclined. Neither 

 action alone causes the cylinder to turn, but when they occur together 

 the cylinder turns, and the number of turns registered on a dial shows 

 with mathematical accuracy how much work has been done. 



In the steam-engine work is done in an alternating manner, and 

 it so happens that this alternating action exactly suits the integrator. 

 Suppose, how^cver, that the action whatever it may be, which we wdsh 

 to estimate is of a continuous kind, such for instance as the continuous 

 passage of an electric current. Then, if by means of any device, 

 we can suitably incline the wheel, so long as we keep pushing the 

 cylinder along, so long will its rotation measure and indicate the 

 result ; but there must come a time when the end of the cylinder is 

 reached. If then we drag it back again, instead of going on adding 

 up, it wnll begin to take off from the result, and the hands on the dial 

 will go backwards, which is clearly wrong. So long as the current 

 continues so long must the hands on the dial turn in one direction. 

 This effect is obtained in the instrument now on the table, the electric 

 energy meter, in this way. Clockwork causes the cylinder to travel 

 backwards and forwards by means of what is called a mangle motion, 

 but instead of moving always in contact with one wheel, the cylinder 

 goes forward in contact with one and back in contact with another 



