November 15, 1895.] 



SCIENCE. 



641 



around the periphery of a mahogany disc, 

 above which is another disc carrying a dial 

 graduated in degrees of temperature. The 

 movable contact which bears on the slide 

 wire is attached to a radial arm placed 

 directly under a hand on the dial, the two 

 being moved together by turning an ebonite 

 knob m the center of the dial. This indi- 



the reading of the instrument, for being 

 made of one piece of metal which has the 

 same temperature throughout its length, it 

 will rise or fall in resistance at the same 

 rate on both sides of Y as its temperature 

 changes, and consequently the ratio of CY 

 to DY will not vary. The effect of tempera- 

 ture changes on the leading wires will not 



cator is enclosed in a brass case attached 

 to the box containing the battery, tele- 

 phone, etc., the box being about 7 inches 

 square and 10 inches high, and furnished on 

 the outside with binding posts for the recep- 

 tion of the leading wires. 



It is easily seen that the temperature of 

 the slide wire has absolutely no effect upon 



sensibly affect the readings because the two 

 wires L and L' are on opposite sides of the 

 bridge, and consequently balance each other. 

 Compared with the resistances A and B 

 these leading wires are of large size, and in 

 order that they may have the same average 

 temperature they are twisted together and 

 covered with braided cotton. 



