On a Simple BJieostat. 173 



The correction for stretchiiio- is 



wc- . 1 ^ ws . c 



= weight of chain x .^s— 



For the ten-chain steel tape the correction is 

 • 0005653 X 7920 x 31:>60 x 8 x 60 

 •1617x3x10" 

 = 1*757 inches. 

 For the ten-chain circuhir wire (Jadern^s) the correction is 



-Oix 2 X (39 60)^^x2-20-1 6 



'39-37 x -8190 x 3 x 10' x 3-1416 x (1-33 x •03937)2 

 = 0-33182 inch. 



In any case the correction depends on the area o£ the cross- 

 section of the wire, bnt not on its form. 



XIX. 0?? a Simple BJieostat. By G. F. C. Searle, M.A., 

 University Lecturer in BJiysics, Demonstrator in Experi- 

 mental Bhysics. Cavendish Laboratory. Camhridae'^. 



1. TX many electrical experiments it is necessary to adjust 

 X the electric current flo^^ing in a circuit to some de- 

 finite value, and this is most easily done by varying the 

 resistance of the circuit. But for accurate adjustment the 

 resistance must be capable of continuous variation, and not 

 merely of variation in steps, as is the case with a resistance- 

 box. This continuous variation of the resistance is generally 

 brought about by making a gradual alteration in the length 

 of a piece of german-silver or platinoid wire included in the 

 circuit, and the object of the present communication is to 

 describe a rheostat in which the alteration is secured in a 

 simple and effective manner. 



The figure shows the general appearance of the rheostat. 

 At the top of an upright there is a wooden wheel with a V- 

 groove in its edge, the wheel turning about a horizontal 

 axis, w^hile at the foot of the upright there is a U-tube of 

 glass containing a little mercury. The total length of the 

 U-tube is about 30 cms. and the internal diameter of the 

 tube is about -8 cm. An endless band about 165 cms. in 

 length, which passes over the wheel and through the U-tube, 



* Communicated by the Author. 



