Messrs. Ayrton and Mather on Galvanometers. 443 



fall. I had a special galvanometer-apparatus made to fit my 

 large magnet, converting it probably into the largest galva- 

 nometer of the type extant ; but the sensibility is exceedingly 

 small when the magnet is fully excited, and increases rapidly 

 when the excitation is somewhat diminished.'" 



The preceding is gravely given by Sir David Salomons as 

 an illustration of the displacement of the current in a con- 

 ductor by a magnetic field " generally known under the name 

 of the Hall effect," in apparent ignorance of the fact that the 

 " Hall effect " is extremely minute. Indeed, had Sir David 

 Salomons tried to compare the current sent by a given P. D. 

 through a coil of insulated fine copper wire when placed in 

 and out of any magnetic field he would have found it practi- 

 cally impossible to detect any difference, and he would have 

 convinced himself that the great falling off in sensibility of a 

 d'Arsonval galvanometer as the strength of the stationary 

 magnetic field was increased beyond a certain limit could 

 have nothing to do with the u Hall effect." 

 . Further, this phenomenon, which is described in the Phil. 

 Mag. for September as new, has for some time past been 

 known to electrical instrument-makers, for it was one of the 

 causes which prevented the sensibility of the suspended-coil 

 type of galvanometer being increased beyond a certain limit. 

 In the discussion which took place after the reading of 

 a paper on a "Workshop Ballistic Galvanometer " before 

 the Physical Society in June 1892, reference was made — 

 perhaps for the first time publicly — to the difficulty ex- 

 perienced in increasing the sensibility of a d'Arsonval 

 galvanometer by increasing the strength of the field beyond 

 a certain point. And it was the investigation of the cause 

 of this phenomenon, and the discovering of a means to over- 

 come it, that caused the publication of the description 

 of this instrument to be delayed until the meeting of the 

 British Association in 1895, as was explained in the account 

 of this latter instrument given in all the electrical journals 

 about September 1895. 



In the 'Electrical Engineer' for October 5th, 1894, it was 

 mentioned that " great difficulties were, however, found to 

 arise when the extreme sensitiveness sometimes required 

 in the laboratory was attempted . . . chiefly due to the 

 traces of magnetism found to exist in the silk and other 

 parts of the coil usually considered non-magnetic .... 

 A great number of trials was made at the Central Technical 

 College which showed that .... tbe purest wire of electrolytic 

 copper or of fine silver, specially drawn and covered, still 

 showed traces, and a method . . . was at length devised which 

 has practically solved the difficulty. The non-magnetic pro- 



