462 Prof. R. Threlfall on the Measurement 



to use, for it was hardly possible to distinguish the motion 

 due to the electromagnetic forces from the never-ceasing 

 motion due to air-currents. Though the instrument was well- 

 protected by a glass case, and this generally supplemented by 

 a wooden box, I never succeeded in eliminating the effect of 

 air-currents, though it must be added I never succeeded well 

 enough with it in other respects to make it worth while to 

 apply the " subjective " method of mirror observation. In 

 any case it seems to be essential to have a means of adjusting 

 the coils to the magnets as well as the magnets to the coils; 

 but the adjustments are very tiresome even with the facilities 

 which in the later forms of this instrument I had for making 

 them. My coils had not quite so much wire as those of the 

 Messrs. Gray because I used all I had and could get no more 

 in Australia. However, the aggregate number of turns 

 amounted to 59,900, and the resistance at 20° C. was 15,852 

 B.A. units as against 62,939 turns, and a resistance of 30,220 

 ohms attained by the authors quoted. The authors also state 

 that the wire was approximately uniformly distributed 

 throughout their coils, though in my case this was found im- 

 possible, keeping the external dimensions quite constant, and 

 therefore there was a slight difference between the coils, 

 which, however, was compensated for by their arrangement in 

 the instrument. I do not think that the diminution of the 

 number of turns had much effect, because the diameter of the 

 coils had reached 5*8 centim. Feeling that my non-success 

 was probably to be traced to my inferior skill as an experi- 

 mentalist, I undertook a long series of trials with a view to 

 discovering the best way of hardening the magnets and their 

 best position in the coils when at rest ; amongst other ex- 

 periments the following will do for description. Three bits 

 of Stubb's steel wire were carefully cut and filed to a uniform 

 length of 4^ in., their diameters being -^ in. These will be 

 called A, B, and C. A was made glass-hard throughout ; B 

 was hardened through a distance of from f to 1 inch at each 

 end ; C was hardened from a distance of from J to ^ inch at 

 each end. These bits of wire were then magnetized by being 

 placed between the massive poles of a very large electro- 

 magnet. The cores of this magnet were 3 inches in diameter 

 and about 25 inches long. The pole-pieces were very broad 

 and thick. The magnetization of the steel was carried nearly 

 to saturation, and the magnets were found by filings to be 

 free from consequent poles. On taking the times of vibration 

 the following numbers were obtained : — 



