NATURAL PHILOSOPHY. 



153 



the idea conveyed in the term " turning on the sun " whenever we 

 need additional light. 



Filially, we have in the new machine a remarkable illustration 

 of the co-relation of the forces the muscular power of the human 

 arm being ultimately converted into a brilliant light, as exhibited 

 by the following chain : 



Muscular power- 



Magnetism: 



Heat 

 Chemical action- 



:Motion 

 ^Electricity 



-Light 

 British Journal of Photography. 



THE COST OF ELECTRIC LIGHT. 



The time appears to be near at hand when the electric light 

 will be used for a variety of purposes. It is worth our while to 

 inquire as to its cost. The expense and inconvenience attendant 

 upon the production of electricity upon a large scale have hitherto 

 been an obstacle in the way of using the electric light, except for 

 lecture rooms and a few other purposes. But the recent improve- 

 ments in the construction of magneto-electric machines and ther- 

 mo-electric batteries have put it in our power to command the 

 services of this beautiful illuminating agent on any desirable scale 

 of magnitude. 



In order to examine the question of cost intelligently, let us 

 refer both electrical and illuminating effects to the common meas- 

 ure of power, namely, the foot-pound per minute. The experi- 

 ments of Mr. Julius Thomson, of Copenhagen, have shown that 

 the power to maintain the light to that of a standard candle for 

 one minute is equal to the raising of a weight, not exceeding 13 

 pounds, one foot high in that time. I have arrived at a similar 

 result from a reduction of recorded experiments made by Miiller, 

 Ritchie, myself, and others. I am satisfied that, where an electric 

 light of not less than 800 to 1000 candles is produced, under prop- 

 er management, the power required will not greatly exceed 15 

 foot-pounds per minute per candle. For smaller amounts of light 

 the power required will be greater. 



Now let us inquire what amount of electricity is the equivalent 

 of, or is represented by, 15 foot-pounds per minute. If 100 feet of 

 No. 18 pure copper wire be coiled into a helix and immersed in a 

 pound of water, and if the ends of this wire be connected to the 

 poles of one cell of the Grove battery (pint-cup size as used in 

 telegraphing), the temperature of the water will begin to rise at 

 the rate of 1 F. in 9 minutes, or 0.10,3 per minute. Now, if 

 the temperature of one pound of water be raised 1 F. per 

 minute, this effect will be the thermal equivalent of 772 pounds 

 raised one foot high in space per minute ; the heating effect, then, 



