CHOICE OF CONDUCTORS. 689 



is then greater, which increases the cost of installation, and more- 

 over the metal rapidly deteriorates, which necessitates a higher 

 rate of redemption. 



These considerations do not hold for wires covered with insu- 

 lators ; the cost of insulation increases in. that case the value of 

 the conductors, and the use of copper is then justified. 



In order to determine the density of the current numerically, 

 the cost of the energy must be estimated. The kilogrammetre is 

 worth 9'8i.io 7 units of work (612) C.G.S., and one horse-power 

 can produce 75 x 9-81.1 o r = 74.1 o 8 units of work per second. 



If C is the price of a horse-power working continuously for a 

 year, the price of unit work is 



which gives 



74- io 8 ' 



74.io 8 

 The most advantageous current then becomes 



i=- = / Q/74 - Io8 =I0 s HQ 



The specific resistance of copper is about 1600. Assume 

 that the price of copper is 175 francs per kilogramme, or 0*0156 

 per cubic centimetre, and that the price for a horse-power-year is 

 250 francs. The electromotor being produced by mechanical means, 

 let us suppose that the current is used uninterruptedly, and that 

 the redemption is io per cent; we shall then have /= io, /= i, 

 which gives 



W : 



74 x 0-156 



3-25 x r6o = 5 37 ' 



The current. will be 5-4 C.G.S. units per square centimetre or 

 54 amperes that is to say, -54 amperes per square millimetre. 



We may add that these results only apply to the ordinary 



conditions ; for the cost of installation, and especially the use of 



insulating materials, tends to raise the price Q of unit volume of 



the metal. Moreover, the factor / is less than unity when the 



VOL. n. Y Y 



