14 Mr. C. V. Boys on Quartz as an Insulator. 



Since sin j3=v/(o, this expression vanishes unless #=/3, 

 when it becomes infinite, so that the magnetic force and the 

 electric displacement seem confined to the surface of a cone 

 of semi-vertical angle /3, the vertex pointing in the direction 

 of motion. 



Trinity College, Cambridge, 

 April 24, 1889. 



II. Quartz as an Insulator. By C. V. Boys, A.R.S.M., F.R.S., 



Assistant Professor of Physics at the Normal School of 

 Science, South Kensington* . 



[Plate I.] 



WHEN making quartz threads by the bow-and-arrow 

 process described in the Philosophical Magazine, June 

 1887, I have sometimes noticed that the thread does not reach 

 all the way from the arrow to the bow, but that the end 

 remains suspended in mid air somewhere between. When 

 this is the case the last foot about, then very fine, is usually in 

 the form of an irregular helix. Under these circumstances, 

 if the hand is brought at all near the end the helix stretches 

 itself out, and the end of the thread flies to and attaches itself 

 to the hand. On removing the hand the thread takes its old 

 form; and this may be repeated several times. 



It did' not seem possible to account for this in any way 

 except by supposing the thread to be electrified, though why 

 it should be electrified is not clear. If this is the case then 

 the insulating-power of the thread must be very great, for 

 with the very small quantity of electricity which could remain 

 on a body of such immeasurably small capacity, all trace of 

 charge would escape instantly if the thread insulated no better 

 than glass in the open air. 



I therefore thought it would be interesting to see if rods of 

 fused quartz showed any great superiority over similar rods 

 of glass under the same circumstances. The plan that I have 

 followed has been to hang a pair of very narrow gold leaves 

 from the rod under examination, and observe the rate at 

 which they closed after being charged. I purposely avoided 

 all instruments the large capacity of which would increase the 

 time of discharge, and the leakage of which might be com- 

 parable with or even exceed that of the rods to be tested. 



The arrangement of leaves &c. is shown in fig. 1. A flat brass 

 hook, A, is fastened to a rod which can slide stiffly through the 



* Communicated by the Physical Society : read April 13, 1889. 



