Prof. Ayrton and Mr. Mather on Galvanometers. 359 

 Table II I 



Description. 



Values of 



r. 



D/r" . 



!)/>•!. 





60 

 40 

 93 



86 



140 



5,800 

 4,280 

 2,200 

 750 

 675 

 600 

 470 



8,750 

 6,150 

 2,730 



1,050 



770 







Weiss' ,, 









Elliott's Galvanometer (from Table I.) 



6,000 



206-5 



493 



For the moving- coil instruments, the numbers are 



Description. 



r. 



D/rK 



D/ri. 



Ayrton-Mather Galvanometer 



243 

 267 

 244 



570 

 421 

 375 



985 

 735 

 650 









Invariable Sensibility Galvanometer l 

 (from 1890 Table) J 



21 



19-8 



27 



These increases in the factors of merit have been brought 

 about chiefly by reducing the dimensions of the suspended 

 parts, and the use of better magnet steel has probably con- 

 tributed something to the improvement. 



The three moving-coil instruments which have surpassed the 

 highest values of D/r* and D/r% given in our 1890 table are 

 of the narrow coil type, the cross-section of the winding being 

 of the form described before this Society in 1890 *, viz., two 

 equal circles in contact. 



To attain the results here shown (Table III.), the transverse 

 dimensions of the coils have been made smaller than usual and 

 very powerful permanent magnets employed. When we 

 compare the records of these instruments with the best listed 



* "On the shape of Movable Coils," &c, Proc. Phvs. Soc. vol. x. 

 p. 376. 



