an Electric Micrometer* . 541 



i. e., (1) when the apparatus was suspended from the door- 

 springs the amplitude was 1/i/x; (2) when the apparatus was 

 unhooked from the springs and rested on the other layers, we 

 had 1(V ju, and so on. 



The above measurements show that solid rubber is more 

 useful than the hollow rubber balls (whose elasticity depends 

 greatly on compressed air), and also that little advantage is 

 obtained by the second layer of balls. 



3. Covers* 



Temperature-changes in the levers, and contacts, may arise 

 in many- ways, but especially from draughts of air and 

 radiation. 



To provide against these, and also against moving dust in the 

 air, different wooden covers were placed over (1) the contacts, 

 (2) the levers, (3) the telephone-receiver and its enclosing 

 metal cylinder. After some trial these precautions were 

 found inadequate ; so the whole apparatus was wrapped in 

 thick felt. This covering served well, not only in main- 

 taining a constant temperature but also in damping sound- 

 vibrations, which in some cases, before its application, produced 

 a chattering of the contact of about the frequency of the sounds 

 themselves. By taking such extensive precautions, immunity 

 from these disturbances was obtained, even in the extreme 

 case when very many gas-jets were burning in the room. 



As regards dust, it was anticipated that great trouble would 

 arise, for even small particles suspended in the air would be 

 large compared to the smallest distances to be measured by 

 the apparatus : such particles would seem likely to remain 

 suspended in the air about the contacts, even after the covers 

 had been put on and the air had remained calm for a long- 

 time. Also we might suppose that the contacts, being elec- 

 trically charged, would be centres of dust collection, so that 

 in fact we should have a clogging of the contacts from dust, 

 just where it was desirable that they should be specially 

 clean. 



To avoid these baneful effects, a special atmosphere round 

 the contacts was provided ; air was driven at a uniform rate 

 through sulphuric-acid bottles, then through cleaned cotton- 

 wool, then through a helical tube in a water-bath, then into the 

 compartment containing the contacts. It was hoped that the 

 contacts would thus have round them dry and dust-free air 

 of uniform temperature. But this plan was soon abandoned, 

 for this special air was found to produce comparatively rapid 

 and great temperature-fluctuations at the contacts ; so that 



