ELECTRICAL UNITS OF MEASUREMENT. 131 



be, a one-millimho lamp, to give a ten- or twelve- 

 candle light with the Board of Trade regulated 

 200 volts of potential. Thus the lamp-galvano- 

 meter, or lamp-counter, may have its scale divided 

 to one millimho to the division, and the number 

 read on its scale at any time will be simply the 

 number of lamps lighted at the time. 1 The in- 

 strument will also have the great advantage of 

 being steady, notwithstanding the variations of 

 the engine. A potential instrument on an electric- 

 light circuit at best is always somewhat variable, 

 because the potential varies a good deal within 

 one or two per cent, perhaps ; but the resistance 

 in the lamps varies exceedingly little. The mho- 

 meter will in these circumstances be an absolutely 

 steady instrument ; you will not see it quiver, even 

 though the engine is irregular. The potential 

 galvanometer will show you how much unsteadiness 

 there is to be complained of or to be corrected. 



1 [Note of December 8, 1887. A form of magnetostatic tangent 

 galvanometer, which I have recently brought out for practical use, 

 serves the same purpose. It is of simpler construction and more 

 convenient form than the mhometer referred to in the text. W. T.] 



K 2 



