of High Specific Resistances. 459 



carrying the wire were also made as much alike as possible, 

 and "the same number of turns of wire were put on each 

 bobbin by means of a revolution-counter. The winding was 

 quite uniform, No. 18 B.W.Gr. wire being used. As a check 

 the resistances of the bobbins were measured, when it 

 fortunately happened that two were about one half per cent, 

 higher than the other two, and so they were paired. The 

 wire, it need hardly be added, was wound on to the four 

 coils under a constant strain. The two electromagnets 

 were then mounted on a permanent stand — one being kept 

 steadily in a fixed position, and the other being capable of 

 sliding parallel to a line drawn perpendicular to the lines 

 joining the centres of the poles of each magnet. The pole- 

 pieces were bevelled off from the top side ; but the area of 

 the ends remained large compared with the size of the mag- 

 nets to be magnetized. The condition as to equality of 

 quantity and quality of steel in the galvanometer magnets 

 was next considered. After some reflexion I decided that the 

 most probable way of securing equality would be to discard 

 bar-magnets entirely and use disk-magnets. I therefore 

 procured a small piece of sheet steel about as thick as 

 ordinary thin writing-paper, and had a die constructed so as 

 to stamp small disks from this sheet. The sheet was fairly 

 hard, and it was found that the disks " stamped" better when 

 the sheet was taken in its natural state than when it was 

 softened. A considerable number of disks were stamped out 

 of the sheet, and these were then laid on a bit of flat iron and 

 raised together to a bright red heat ; they were then plunged 

 together into a jar of cold water. On examination they all 

 seemed to be glass-hard, and some of them remained flat. 

 The four flattest ones were chosen and prepared for mounting. 

 A bit of aluminium wire was cut to the right length and 

 beaten out flat at each end. The disks were then cemented 

 with shellac varnish, one on each side of each flattened end 

 of the aluminium wire. The wire was thus much more ac- 

 curately the centre of rotation of the magnetic system than is 

 generally the case. Attempts were then made to get a good 

 light mirror. About three ounces of small microscope cover- 

 slips were examined by aid of the reflected image of the bars 

 of a window, and from these about twenty were selected 

 and silvered by the Rochelle salt process. They looked very 

 good, but on mounting for trial without strain they all turned 

 out disappointing. I finally made use of a small portion of a 

 larger mirror that had got broken. This was cemented on to 

 the flat surface of one of the steel disks and was found not 

 to be sensibly distorted. After trying very many cements, I 



