556 Mr. F. E. Smith on Bridge Methods for Resistance 



to several ohms, it is necessary to describe the construction 

 of such a resistance in some detail. Messrs. Dickinson and 

 Mueller * have already described a system capable of such 

 changes, and a resistance-box based on that system was 

 exhibited to the author by Messrs; Waidner and Burgess of 

 the Thermometric Department of the Bureau of Standards, 

 Washington. The author here expresses his thanks to 

 Messrs. Waidner and Burgess for the great kindness shown 

 to him on that and other occasions. 



The system of resistances which we describe here has not, 

 we believe, been previously used. Fig; 5 shows the arrange- 

 ment. All the coils are of mangamn, and those t)f O'Ol ohm 



Fig. 5. 



X O'OOl x O'OOOI x O'OOOOI 



Variable Resistance. Method 1. 



I'he r^sifetance 'coils in the three dials are as follows : — for the O'OOl ohm 

 dial, ten resistances of O'l ohm each and ten of 9*9 ohms each ; for the 

 0-0001 ohm dial, ten resistances of 0*05 ohm and ten of 24*95 ohms ; and 

 for the 0-00001 ohm dial, ten resistances of 0'0l ohm and ten of 

 S-99 ohms. 



and upwards (apart from the dials) have mercury cohtact- 

 j)ieCes, after the manner of those introduced by the Cambridge 

 Scientific Instrument Co. The three dials are used to 

 produce the small changes of resistance. In the case of the 

 0*001 ohm dial we have It) resistances of 0*1 ohm each in 

 series, and 10 resistances of 9*9 ohms each, also in series. 

 The 'contact-brushes enable one or more of the 0*1 ohm 

 resistances, each to be shunted by 9*9 ohms, and the shunting 

 of each coil produces a diminution of the total resistance of 

 O'OOl ohm. This device does not involve any very accurate 

 adjustment of the coils, as will readily be seen. If the 

 resistances are exactly O'l ohm and 9'9 ohms respectively, 

 then when placed in parallel they will produce changes in 

 the resistance for each step of the dial of exactly 0"001 ohm. 



* Dickinson & Mueller, Bull. Bur, of Stands, vol. iii. no. 4, p. 646 

 (1907). 



