90 Mr. C.V. Boys on Apparatus for the 



no objection to such a course ; but as a rise in temperature 

 makes an electrolyte a better, and a metal a worse conductor, 

 any electrolytic meter combined with a shunt would have a 

 tendency to show too much in warm weather or if warmed 

 by the current. If, again, there is any polarization, and that 

 polarization is not strictly proportional to the current, then 

 another error will be introduced. It would seem therefore 

 necessary, if accurate indications are required, to make the 

 whole current pass through an electrolytic meter. 



Electric-Energy Meters. 

 Since the energy expended by an electric current between 

 any two points is equal to the current multiplied by the dif- 

 ference of potential of those points — that is, to the main current 

 multiplied by a shunt current passing in a wire of high resist- 

 ance between the two points — an electric-energy meter may be 

 made by combining two electric-current meters, which take 

 account of the direction in which the electricity passes, in 

 such a way that the first integrates the main-current strength, 

 and that the mangle-motion of the second is driven by the 

 integrating-cylinder of the first : then, if the needle of the 

 second is deflected by the shunt current, the rotation of the 

 second cylinder will give the integral of the energy expended; 

 This is obvious ; for the rate of rotation of the second cylinder 

 is proportional to its rate of reciprocation multiplied by the 

 tangent of the inclination of its tangent- wheel — that is, to the 

 strength of the current in the first machine multiplied by the 

 strength of the current in the second ; so its rate of turning 

 is proportional to the rate at which energy is being expended, 

 and its whole rotation is a measure of the total energy. If at 

 any time either the main or the derived current, but not both, 

 changes sign, then the second cylinder will begin to turn the 

 other way, showing that the current is not doing work in the 

 portion of the conductor between the points, but is being 

 caused to flow by an action of some kind taking place in that 

 portion. If both currents change sign, then, as before, work is 

 being done ; and though the first machine is working back- 

 wards, the second is working forwards. A combination of 

 two machines, as described, would integrate both the current 

 and the energy. A more simple and practicable machine for 

 integrating energy alone is shown in figs. 11 and 12. The 

 integrating mechanism has been already described; the elec- 

 trical principles employed must now be explained. If two 

 wires, one conveying the main current and the other the de- 

 rived current, are near one another, they will attract or repel 

 one another with a force which is proportional to the product 



