434 Mr. Norman Shaw on Increased Accuracy 



suspensions, but these difficulties have caused it to fall some- 

 what into disuse. It is the purpose of this paper to suggest 

 a method for treating the bifilar suspension in order that it 

 may be more successfully employed for observations involving- 

 very refined measurements. 



The investigation has been performed in connexion with a 

 research on the absolute measurement of the electromotive 

 force of the Weston standard cell by means of the Weber 

 electrodynamometer. This instrument was purchased by 

 McGrill University as a part of the general equipment of the 

 Macdonald Physics Building about 1893. It was carefully 

 made by N alder, and was an exact copy of the dynamometer 

 constructed by the Electrical Committee of the British 

 Association, and described by Maxwell*. The instrument 

 was setup by Mr. R. 0. King in 1895, under the direction of 

 Prof. Callendar. The work by Mr. King has been fully de- 

 scribed by Callendar t, who made some important changes in 

 its windings. Preliminary observations were obtained by Mr. 

 King for the electromotive force of the old Board of Trade 

 form of crystal Clark cell, and it was intended that more 

 accurate work should be obtained later. This work was not, 

 however, carried out, and the dynamometer remained in 

 disuse for over ten years. It was suggested by Prof. Barnes 

 that the writer should complete this investigation, and through 

 the genero-ity of Mr. King, a Fellowship was provided, 

 which enabled him to devote a } T ear almost without inter- 

 ruption to the work. It was found, during the series of 

 observations on the deflexion measurements, that there was 

 a serious difficulty due to a small elastic after-effect, which 

 rendered this part of the work less accurate than the other. 

 In order to study this more carefully, the writer devoted a 

 considerable amount of time to determining the effect of 

 repeated deflexions on the true reading of the instrument. 

 It was found possible to systematize the results in a satis- 

 factory manner, and work out mathematically the equations 

 for determining the correction to be applied in any particular 

 case. In many ways this was an unusually good opportunity 

 to test the constancy of bifilars. The suspended coil had 

 been hanging for over ten years, and it was thought that the 

 wires would be as permanent in their behaviour as any of 

 the kind could be expected to be. They were made of hard 

 drawn copper, diameter '0450 cm., length 80 cm., and were 

 rigidly clamped 3'1 cm. apart. 



This suspension fulfils remarkably well all the requirements 



* 'Electricity and Magnetism,' (3rd ed.) vol. ii. p. 367. 

 t Phil. Trans. A. vol. cxeix. 1902, p. 55. 



