468 On the Measurement of High Specific Resistances. 



same weight as nearly as possible as the astatic combination 

 to be examined, was suspended from the fibre in a vibration- 

 box furnished with a long glass tube. The copper ultimately 

 employed was supposed to be electrolytically pure. The 

 combination at first was slightly diamagnetic, but became 

 much less so as the paint-cement dried, and was finally almost 

 indifferent to any means we could find for testing it. From 

 experiments with this disk, whose weight and moment of 

 inertia were known, the coefficient of torsion of the fibre was 

 found to be T = -000115 C.G.S., with a load of '287 gram. 



Two astatic combinations were next mounted and tested. 

 The first was the one that had already done some work in the 

 galvanometer, the second was carefully made for the purpose. 

 The moment of inertia of the first was found to be '02126, 

 and of the second -03274. 



With both combinations two sorts of experiments were 

 made. The time of vibration of each was determined, and 

 the change of zero produced by twisting the upper end of the 

 fibre through a known angle, generally 2-7T. From these 

 well-known methods it was found that the systems had a 

 period of vibration of about 2'51 seconds only. This corre- 

 sponded to a value for the moments of the forces of about 

 '1270. The magnitude of this number, as well as the positions 

 taken up by the combinations, showed not only that the asta- 

 ticism was far from perfect, but also indicated the cause of 

 this : the magnets were not really in one plane. Tentative 

 twisting of the aluminium wires was then resorted to, with 

 the result of bringing up the periods of vibration to 11 and 

 12 s 8 seconds respectively. In this latter case the moment 

 was reduced to '0064. The needle that had been brought to 

 a free period of 11 seconds was mounted in the galvanometer, 

 and by means of the controlling magnet was brought to have 

 a period of 36 seconds, corresponding to a magnetic moment 

 of about '0007 C.Gr.S. During the experiments on resistance 

 the period was got considerably longer than 35 seconds. 



The next paper, on the Resistance of Impure Sulphur, 

 contains the details of the method employed to find both the 

 specific resistance of the gums mentioned and of sulphur. As 

 no useful result is to be expected from a publication of the 

 long series of experimental numbers obtained in the work on 

 Gums, I refer to the following paper for the description of 

 the method employed, since it remained constant throughout. 



