PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 453 



ter and put the two magnets in its place, both connected in the 

 same circuit with the rheostat. Let the magnets be adjusted Avith 

 equal fineness, and then gradually switch in resistance until one or 

 both shall fail to operate. If there is diiFereuce in them the better 

 magnet will continue to work after the other shall have ceased. 



An interesting; discussion followed, in the course of which Dr. 

 Bradle}^ said he had taken as his unit that adopted by our coun- 

 trymen. Gen. Lefferts, and Moses B. Farmer, who had succeeded 

 in producing a very compact and convenient form of rheostat, 

 using spools of fine wire, the passage of the current through 

 which was equal to any desired length of line. The doctor had 

 arranged a series of these, adjusted with great care in a single 

 compact instrument, provided with a delicate slide, lever, switches, 

 etc., whereby a greater or less number could be turned on, so as 

 to attain any desired amount of resistance, and show exactly what 

 is the length of line equal to that resistance. 



Some instruments for receiving and recording; the sig-nals used 

 in telegraphing produce a resistance equal to ten or twenty miles 

 of line, while others are so constructed or adjusted as to induce 

 a resistance equal to 100 miles or more. The instruments with 

 great resistance work with a weaker current, and are all adapted 

 for certain situations. 



The instruments having coils of slight resistance should be 

 arranged along the middle of the line, and those of great resist- 

 ance at the ends. This apparatus would measure and assort them 

 with great facility. 



Hill's Battery. 



A great simplification of battery, lately invented by Dr. E. A. 

 Hill, of Chicago, was also exhibited. This dispenses with the 

 porous cup heretofore considered necessary between the two solu- 

 tions. It had been found that the difference in the specific gravi- 

 ties Avas sufficient to keep them separate with proper care. Instead 

 of arranging the zinc plate around the copper plate with the 

 porous partition between, as in the popular Grove battery, Dr. 

 Hill uses flat plates located one above the other with no partition 

 between. The copper plate lies near the bottom of the cup, the 

 zinc plate near the top. The cup is nearly filled with sulphate of 

 zinc, and allowed to become quiet. The sulphate of copper is 

 then gently introduced in the form of solid crystal which falls to 

 the bottom of the cup and there soon dissolves. The result, me 



