THOMSON'S ELECTROMETER AND GALVANOMETERS 67 



I devised a floating conductor of glass tube full of mercury to replace the copper wire. 

 The mercury is so much worse a conductor than copper, that it required four cells 

 to give a good effect. 



6 GREENHILL GARDENS, 



EDINBURGH, 18/12/61. 

 My dear Andrews, 



I find that I cannot manage to visit Belfast at present my simple reason 

 is that I am to bring home from Glasgow (where I am going to stay a day with 

 Thomson) two galvanometers and an electrometer on Saturday next and I must 

 have one galvanometer and electrometer fitted up during the holidays, as I shall just 

 have reached the critical point of Radiant heat when we stop. The new galvanometer 

 works by reflexion, and can therefore be easily shown to a large class, which was 

 impossible with the needle ones besides it is delicate enough to show an effect even 

 by frog-currents. 



The electrometer also works by reflexion, and gives a deflection of some inches 

 on a scale for ^th of the electromotive of one cell (Daniell). Of course the gold- 

 leaf electroscope must now remain unused on the shelf, or at most be brought out 

 to show what we used to be content with.... 



This prophecy of Tait's was not fulfilled even by himself during the suc- 

 ceeding forty years of lecturing. There is a simplicity about the gold-leaf 

 electroscope which will ever keep it a prime favourite for purposes of demon- 

 stration, especially now when it is so easy to project the moving and divergent 

 leaves magnified upon a distant screen. 



GREENHILL GARDENS, 



EDINBURGH, Jan. 15, 1862. 

 My dear Andrews, 



Three reasons especially urge me to write to you to-night the first and 

 most pressing I shall detail at once. 



I wish to know (by return of post if possible) what is the nature of the new 

 ammonia process for procuring cold, and from whom, and at what price, it can be 

 procured. This urgent business having been got over, I can be more easy in my 

 future remarks. 



You should at once get William Thomson's galvanometers acting by reflexion. 

 I have been lecturing on heat for some 4 weeks back ; and I have shown, to my whole 

 class, not only Melloni's experiments about diathermancy &c., but on a large scale the 

 polarization of dark and bright heat... 



Next I wish to know where your (and others') results as to Heat of Combination 

 are to be found. 



As to myself I may say that I have done nothing experimentally for a long time 

 except with a view to familiarising myself with new apparatus.... The beauty of the 

 new galvanometers is such that today I arranged to show in a future lecture the 

 Inductive Effects of the Earth's magnetism on a coil of wire about 30 feet long, coiled 



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