94 On the Measurement of Mechanical and Electrical Forces. 



into the secondary circuit resistance-coils. To prevent waste 

 of clockwork energy or of electricity, the main current is made 

 to pass round a subsidiary electromagnet whose armature 

 allows the clockwork to go only when the main current is 

 passing. This armature also, on being attracted, completes 

 the secondary circuit, so that the derived current can only flow 

 when the main current is passing. 



The meter may be used as an energy measurer or indicator 

 (not meter) with advantage when setting the carbons of an 

 electric arc. If the two poles are made to touch, though the 

 current is enormously increased, the energy is reduced, owing 

 to a greater falling-ofF in the difference of potential between 

 the poles. As the carbons are gradually separated the current 

 diminishes, but the difference of potential increases in a higher 

 ratio; so their product increases; that is, the energy expended, 

 and so the heat and light produced, increases. This increase 

 is shown by an increased inclination of the beam. After a 

 time a point is reached at which the current decreases in 

 the same ratio that the difference of potential increases; at 

 this point the inclination of the beam attains a maximum ; 

 beyond this the decrease of the current is in a higher ratio 

 than the increase in the difference of potential, so the energy, 

 heat, and light fall off, as is indicated by the diminution of 

 the inclination of the beam. If, therefore, the carbons are 

 placed so that the inclination of the beam is a maximum, then 

 the best effect is being obtained. In the same way, if the cur- 

 rent is being employed to drive a machine, the most effective 

 speed for that machine may be found by observing at what 

 speed the inclination of the beam is greatest. 



The various meters described depend for their numerical 

 results on two things — (1) the horizontal intensity of the 

 earth's magnetism, (2) the force of gravity. The indications 

 of the first current-meter are inversely proportional to the 

 horizontal intensity, and of the first energy-meter to the 

 square of the horizontal intensity. Each of them is dis- 

 turbed by changes in the direction of the earth's magnetism. 



The second current-meter and the second energy-meter are 

 independent of the magnetic field altogether. Their indi- 

 cations vary inversely as g when the clockwork is regulated 

 by a balance-wheel, or inversely as \J g when a pendulum-clock 

 is used. This dependence on gravity is a point of very great 

 importance ; for over any one country gravity does not change 

 appreciably, nor does it matter in what direction the machine 

 is placed so long as it is level. By screwing the weights X X 

 up or down, so as to decrease or increase the influence of 

 gravity on the meter, its indications may be regulated to a 



