Prof. Ayrton and Mr. Mather on Galvanometers. 361 



Roy. Soc. 1896, vol. clxxxvii. pp. 80-97), and used by him 

 and Mr. J. H. D. Brearley in their " Researches on the Elec- 

 trical Properties of Pure Sulphur." This instrument is in 

 many respects a remarkable one. Unfortunately the details 

 of the sensibility tests given in the printed paper are hardly 

 sufficient to enable us to make the reductions to constant 

 period and scale distance, as is done in Table II., so we give 

 the results separately below. The tests were made by ob- 

 serving the throw produced on reversing a small current 

 through the galvanometer, the throws being read by a tele- 

 scope at about 267 centimetres from the galvanometer. The 

 telescope had a micrometer eyepiece and a glass scale in it 

 divided to fifths of a millimetre. 



Date. 



Period in \ Current in 

 seconds. 1 amperes. 



Throw on 

 reversal in 

 micrometer 



divisions. 



Amperes per I 

 throw of 

 1 division. 



Oct, 17th, 1892. 

 Sept. 1893 



14-5 

 25 



274xlO- n 

 2-74 x 10" 12 



7-9 

 193 



3-5 Xl0- 12 * ; 

 l-43xl0- 13 t: 



The definition of the optical system was sufficiently good 

 to permit the micrometer-scale being read to a fifth of a 

 division, so that currents of one-fifth the magnitude of those 

 given in the last column of the above Table could be detected. 



Oscillographs. 



Oscillographs, or instruments for showing the character of 

 rapidly-varying currents or potential-differences, were practi- 

 cally unknown when our 1890 paper was read. They have 

 become possible by the great reduction of dimensions of 

 moving parts which has gone on continuously since that date. 

 An oscillograph may be defined as a galvanometer of very 

 short period. The short periodic time is usually obtained by 

 using very small moving parts and a very strong control. 

 In these instruments the reduction of size has been carried 

 far beyond what has yet been attempted for ordinary scale- 

 reading work. Mr, McKittrick's % galvanometer has a 

 moving system whose moment of inertia is about one- 

 millionth of a C.Gr.S. unit, or about one ten-thousandth the 

 inertia of an ordinary light astatic needle fitted with a f in. light 



* Phil. Trans, vol. 187, line 1 of table vi. p. 95. 



t Ibid, line 1 of table vi. p. i)6. 



I Trans. A. I. E. E. vol. xiii. Nos. C and 7. 



