of High Specific Resistances. 467 



The arrangement for adjusting the distance of the control- 

 ling magnet was arrived at after several trials. It consisted 

 of an apparatus sliding and clamping on the brass tube, with 

 a nut and screw for fine adjustment. It was intended to use 

 a worm-wheel and screw for the adjustment in a horizontal 

 plane ; but this was found after a little practice to be un- 

 necessary, although it would be convenient. 



The details of this arrangement for raising and lowering 

 the magnets will be understood from the drawing of the gal- 

 vanometer for medical purposes on PL XIY. figs. 1 and 2, see 

 p. 416, supra. Many experiments were made in order to decide 

 the relative merits of quartz and silk fibres. At first it was 

 thought that silk did as well ; but after a time a great deal of 

 trouble with the zero was traced to the silk, and attempts were 

 made to use finer quartz threads. In this, owing to the skill 

 acquired by Mr. Pollock, I was finally successful. As I have 

 had about a year's experience in drawing quartz threads, I add 

 the following notes on the process in hope that they may 

 prove of use to others. The difficulty is to get a large enough 

 bit of quartz fused onto a suitable handle. The best way of 

 managing this is first to heat a bit of rock crystal red-hot in 

 an ordinary crucible and keep it heated for about an hour. 

 On cooling, it will be found to have split into fragments of all 

 sizes ; one of these is chosen, and supported on a bit of lime 

 or on a massive bit of iron, and is then fused under the oxy- 

 hydrogen (not oxy-coal gas) jet. When it has once been got 

 glass-like it never cracks again, no matter how suddenly it 

 may be heated. Porcellanous quartz draws into rotten threads, 

 as might be expected. Two bits of fused quartz having been 

 prepared they may be fused to the ends of two bits of clay 

 tobacco-pipe, and can then be manipulated in the oxyhydrogen 

 flame without trouble. There is no difficulty (when once the 

 short thick threads have been drawn by hand) in the subse- 

 quent shooting. I most cordially indorse all that Mr. Boys 

 says in favour of this admirable invention. 



Another difficulty lies in the obtaining of a reasonable 

 degree of astaticism. It has already been shown that it is 

 practically possible to increase the astaticism of a magnetic 

 combination by careful methods of magnetization and manu- 

 facture of the magnets : but the astaticism thus in general 

 obtained is by no means perfect. The investigation of this 

 matter was undertaken by Mr. Adair, and proved to be diffi- 

 cult and unsatisfactory. In the first place it was necessary to 

 determine the coefficient of torsion of the silk fibre to be used 

 in the experiments. This fibre was about thirty inches long, 

 and before mounting had been boiled in a tube of water. A 

 copper disk, made up with a mirror so as to have about the 



