Prof. Ayrton and Mr. Mather on. Galvanometers. 351 



round number ; hence we decided in 1890 to use this time as 

 the standard period to which all sensibilities were reduced. So 

 far as we can see, this is still the most convenient standard 

 time, and it has therefore been retained in the present paper. 

 The reduction to a standard resistance of one ohm has 

 proved of considerable value io comparing various instruments, 

 and has been frequently used by others interested in the 

 subject. The question as to whether the " square-root law," 



t. e. 



Deflexion per microampere oc ^resistance, 

 or the two-fifths power law, 



Deflexion per microampere oc (resistance)*, 



should be used in this reduction is still perhaps debatable. 

 On the whole, we consider the two-fifths law more nearly 

 true over wide ranges of resistance ; nevertheless we have 

 given the principal results reduced according to both 

 systems. 



Explanation of Tables I. and II. 



Column 1 (or T) gives periodic time of vibration in seconds when tested. 



„ 2 (or A) „ logarithmic decrement of motion when tested. 



„ 3 (or F) „ deflexion in divisions per microampere (as tested) 

 when scale-distance =1000 divisions. 



,, 4 (or r) „ resistance of coil in ohms. 



„ 5 (or M) „ deflexion in millimetres per microampere when 

 undamped period is 10 seconds and scale placed 

 as in actual use of the instrument. 



„ 6 (or D) „ deflexion in divisions per microampere when 

 undamped period = 10 seconds and scale-dis- 

 tance =1000 divisions. 



„ 7 (or S) „ swing per microcoulomb under same conditions 

 as in last. 



„ 8 (or V) ,, volume occupied by convolutions of wire, in 

 cubic centimetres (approximately). 



,, 9 (or I) „ moment of inertia of the suspended system, j n 

 C.G.S. units (approximately). 



„ 10(orDI)„ deflexion in divisions ( scale - distance = 1000 

 divisions ) per microampere for constant con- 

 trolling moments, and for a periodic time equal 

 to 10 Vl seconds. 



„ 11 (orSF) ,, swing per microcoulomb under same conditions 



as in last. 



Columns 12, 13, and 14 give the deflexion per microampere and swing 



per microcoulomb, when the period is 10 seconds 



and resistance of each instrument is one ohm*. 



* Column 12 is based on the assumption that the sensitiveness is 

 proportional to the square root of the resistance, whilst columns 13, ]4, 

 15, &c. assume that sensitiveness varies as the two-fifths power of the 

 resistance. (See Phil. Mag. July 1890, p. 85 et seqq., and Proc. Phys. 

 Soc. of London, vol. x. p. 422.) 



2 C 2 



