360 Prof. Ayrton and Mr. Mather on Galvanometers. 



in our 1890 table (particulars of which are given in the last 

 line of the preceding table) the improvement is evident ; for 

 whereas the highest value of D/Vf given by a moving-coil 

 instrument in 1890 was 27, the present highest value is 985. 



It is also interesting to notice that so far as absolute sen- 

 sitiveness for current is concerned (/. e. the u millimetres at 

 a metre " per microampere, unreduced for period and re- 

 sistance) the progress made with moving-coil instruments 

 during the last eight years has been considerable. In our 

 1890 Table the d'Arsonval of greatest absolute sensitiveness 

 as an ammeter was the " Large invariable sensibility " instru- 

 ment given in line 4 of the d'Arsonval section of that list (see 

 line 34, Table I.). This galvanometer gave 6'1 divisions per 

 microampere, its period being 2*6 seconds, and coil-resistance 

 21 ohms. In the more recent list, Table II., the above record 

 is surpassed by no less than thirteen instruments out of the 

 twenty-one tabulated, the most sensitive d'Arsonval galvano- 

 meter to current being one made by Messrs. JSTalder Bros. 

 & Co., which gives 760 divisions per microampere, its period 

 being 12*9 seconds and resistance 1053 ohms (see line 37, 

 Table II.). 



On comparing column 15 of Table II. with the corresponding 

 one in the 1890 Table, which gives the values of Dl/rf, i. e. 

 numbers proportional to the deflexion per microampere with 

 constant controlling moment "*, we observe that no increase is 

 perceptible. On the contrary, the numbers in the columns of 

 the present table are on the average smaller than in the 

 previous one. This is what might be expected from a 

 diminution of the dimensions of the moving parts of the 

 instruments. An examination of columns 16 in the two 

 tables (values of Sl 2 /^) leads to a conclusion similar to that 

 deduced from a comparison of columns 15, the only instance of 

 increased values being in the three narrow-coil d'Arsonval 

 instruments just referred to. 



It is perhaps worth while again calling attention to the fact 

 of the superior sensitiveness (as ballistic instruments) of gal- 

 vanometers with suspended parts having small inertia, a fact 

 which is clearly shown by a study of columns 9 and 14 of the 

 tables. 



Prof. R. ThrelfalVs a New " Galvanometer. 



Since the paper was read our attention has been directed to 

 a very sensitive galvanometer designed by Prof. R. Threlfall, 

 MA. (an account of which is given in the Phil. Trans, of the 



* See paper on "Galvanometers,"' Phil. Mag. July I860, and Proc. 

 Phys. Soc. vol. x. p. 421. 



