T II E 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



OCTOBER 1898. 

 



XXXVI. Galvanometers. — Third Paper. 

 By Prof. W. E. Ayrton, F.R.S., and T. Mather *. 



Section A. 



Introductory. 



UNDER the above title the authors, in conjunction with 

 Dr. Sumpner, read a paper before the Physical Society 

 in January 1890 t, and gave a list of sensibility-records 

 relating to a large number of instruments of various types. 

 To facilitate comparison between different instruments, the 

 results were reduced to a common standard as regards scale- 

 distance, length of a scale-division, periodic time of vibration, 

 and resistance, the values chosen for this purpose being, 

 respectively, a scale-distance equal to 2000 scale-divisions, 

 periodic time 10 seconds, and resistance 1 ohm. The sen- 

 sibilities for steady currents were expressed as the number of 

 scale-divisions deflexion a current of one microampere would 

 produce (assuming proportionality) under the standard con- 

 ditions, and the sensibilities for quantity as the swing produced 

 by the instantaneous discharge of one microcoulomb when the 

 instruments were used ballistically. The resulting numbers 

 give a measure of the relative sensitiveness of the various 

 instruments when quickness of action is all important. For 

 cases where time is of little consequence, and constancy of 



* Communicated by the Physical Society : read May 13, 1898. 

 t Proceedings of the Physical Society of London, vol. x. p. 393 ; 

 Phil. Mag. vol. xxx. p. 58. 



Phil. May. S. 5. Vol. 46. No. 281. Oct. 1898. 2 C 



