Prof. Ayrton and Mr. Mather on Galvanometers. 353 



ments for constant period and constant resistance of coil, when 

 the galvanometers are used as ammeters. The same columns 

 give their relative merits as voltmeters, provided the resistance 

 of the coil is equal to the resistance between the terminals, 

 i. e. when the resistance of the connexions is small compared 

 with that of the coil. This may be taken to be the case in 

 instruments with fixed coils ; but for moving-coil galvano- 

 meters it is by no means true, especially when the coils have 

 low resistance, for in some instruments the resistance of the 

 suspensions is greater than that of the coil. Further, as 

 instruments of the d'Arsonval type cannot always have their 

 period, and therefore their sensitiveness, easily varied, no 

 reduction to constant period or constant resistance has been 

 made in Table V.,and the numbers in column 3 (F') Table V. 

 give the actual sensibilities of the respective instruments 

 as voltmeters when the scale-distances equal 1000 scale- 

 divisions. 



AVe may again point out (as in our original paper *) that 

 the comparison of galvanometers by reduction to constant 

 period, scale-distance, and resistance, is a purely electro- 

 magnetic comparison, and takes no account of optical magni- 

 fication, which in some instruments is of a much more perfect 

 nature than in others. It is also to be noticed that the Table II. 

 contains some records of galvanometers having very short 

 periods, e. g. instruments numbered 4, 25, 41, 42, 43, 46, 

 and 47; and the results of the tests on these instruments have, 

 for the sake of uniformity, been reduced to the same period, 

 scale-distance, and resistance, as the galvanometers having 

 longer periods. It must not, however, be supposed that these 

 short-period instruments could easily have their periodic 

 time lengthened to 10 seconds — indeed, in some cases this 

 would be practically impossible. But although the values 

 given in columns 5,6, 7, 12, 13, 14, 17, and 18 for the above- 

 mentioned short-period instruments cannot as yet be realized 

 in practice, the ratios of these numbers to those in the corre- 

 sponding column for any other instrument give approximately 

 the relative sensibilities of the two instruments compared, 

 when both have the same short periodic time. To make our 

 meaning clearer we will compare instruments numbered 24 

 and 43 in Table II. Referring to column 13 we find the 

 values of D/r% for these instruments to be 254 and 985 

 respectively, indicating that No. 43 has a " factor of merit " 

 between three and four times as large as No. 24. This must 

 not be taken to mean that No. 43 is actually more sensitive 



* Phil. Mag. vol. xxx. pp. 83-84 ; and Proc. Phy?. Soc. of London, vol. x. 

 p. 420. 



