12 Messrs. C. W. Waidner and F. Mallory on 



method used or the mass of figures involved, it is sufficient 

 to state that the method was in general similar to the one 

 described by Mr. E. H. Griffiths in ' Nature/ Nov. 14th, 

 1895. The figures given below are interesting as showing 

 the degree of accuracy attainable. It will be seen that the 

 two calibrations gave almost identical results. 



Table I. 



Coils. 



Nominal values 



in Box Units. 



[approx. 1 B.U. 



= •01 ohm.] 



Corrections (in mean B.U.) to 



reduce nominal values to mean 



box-units at 20° C. 



Mean. 



1st Cal. 



2nd Cal. 



H 



5 

 10 



20 



40 



80 



160 



320 



640 



-•0009 

 + •0042 

 + ■0312 

 + •0803 

 + -0284 

 -•0415 

 -■0383 

 - -0637 



-•0007 

 + •0036 

 + ■0310 

 + 0799 

 + •0277 

 -•0430 

 -•0362 

 -0635 



-•001 

 + •004 

 + 031 

 +•080 

 + ■028 

 -•042 

 -•037 

 -•063 



G 



F 



B 



D 







B 



A 





FI 



100 



+ •037 





Calibration of Bridge-icire. — This consists in finding the 

 resistance of each centimetre of the bridge- wire in terms of 

 the mean box-unit at some standard temperature. In resist- 

 ance-box No. 7 the scale of the bridge- wire, extending in 

 each direction from the centre, is divided into centimetres, 

 and very approximately 1 cm. of the bridge-wire has a resist- 

 ance equal to 1 mean box-unit. The final results of the 

 calibration are therefore given in the table below as a correction 

 which is added to the bridge-wire reading (in cms.) to reduce 

 this to its equivalent resistance in mean box-units. The 

 process is exactly similar to the calibration of a graduated 

 glass tube, where the length of a short column of mercury is 

 measured in different parts of the tube. A gauge-coil of 

 manganin wire whose resistance was equal to about 2 cms. of 

 the bridge-wire was balanced against different portions of the 

 bridge-wire. Different parts of the bridge-wire (approx. 

 5 cms. long) were next balanced against the H (5) coil of the 

 resistance-box, and a connexion thus obtained between the 

 bridge-wire and box-unit. 



