466 The Attachment of Quartz Fibres. 



13. If it is required to conduct electricity, as for instance 

 to keep the needle of a quadrant electrometer electrically 

 connected with a batteiy, the whole may now be silvered in 

 a long tube and washed, otherwise it will insulate most 

 perfectly. It may be mentioned here that for the most 

 delicate possible electrometer, as I found in my experiments 

 on the pocket electrometer, it is useless to expect to find any 

 stability where a liquid surface is pierced. The only method 

 of communicating with the needle is through a silvered 

 quartz fibre. Owing to the insulating quality of a clean 

 quartz fibre, delicate experiments are apt to be disturbed by 

 unintended electrification of the suspension, and this may 

 still remain after means have been employed to prevent it, 

 for mere metallic contact between different metals leaves the 

 surfaces in effect at different potentials, depending on the 

 metals used, and, as I showed, in an idiostatic instrument 

 the disturbance due to platinum and zinc is many hundred 

 times the least that can be detected. 



I have sought to reduce this form of error by either or 

 both of two methods. In the first I make the inside of the 

 chamber surrounding the suspension a figure of revolution, 

 the axis being the line of the fibre; in the other, when possible, 

 I make the surfaces of the suspension and of the enclosure 

 one and the same, preferably electro-gilt. 



The first method in very small instruments also in the 

 main avoids what can no longer be safely neglected, as it has 

 hitherto nearly always been, viz. gravitational attraction. 



There is one more point which may be of some interest. 

 If an unsilvered quartz fibre is threaded through a small hole 

 in a thin metal plate, stretched by a suspended weight, and 

 the hole is then wetted with chloride of zinc and soldered up, 

 the fibre will, after washing off the fused chloride of zinc, 

 pull out, leaving a hole fine and beautifully circular. 



It is unnecessary to say more than I have already done, on 

 more than one occasion, on the necessity for making the free 

 space round a suspension in any instrument of extreme 

 delicacy as small as possible and enclosing it by massive 

 metal, itself protected from outside heating and cooling by a 

 non-conducting cover, such as I have in the radio-micrometer ; 

 otherwise the convection currents set up in the free space will 

 blow the suspension about, and produce vagaries which might 

 be easily attributed to the fibre or its attachment. The dis- 

 turbances due to this cause are apt to be much greater than 

 anyone would at first imagine, and the small trouble spent in 

 avoiding them in the manner indicated is well rewarded. 



With regard to the manipulation with fine fibres, I have 



