EXPERIMENTAL ACTIVITY 75 



17 D. P. E. 5/7/69. 

 Dear T., 



I have just heard from T" [i.e. Tyndall] that you are in Largs. I feared 

 you would be in a state of suspense and uselessness at Brest. 



Do you mean by multiple-arc coils the set which has a separate frame for plugs 

 one in fact into which plugs are to be put, not out of which they must be taken, in 

 order to work them ? If so I shall send them off at once on hearing from you, for 

 I have not even attempted to work with that set. 



The other set works capitally and I have almost finished my copper wire 

 determinations by its help besides having carefully got the values of the coils of 

 my own set; the unit in which is curiously (purposely?) r$ B.A. units very nearly. 



You did not answer my query about the equation for heat in a bar. Do so 

 now. 



d f, df 



for two similar bars which when heated and left to cool work exactly together Is 

 not k<x (A#) s ? A. is as nearly as possible the same in both. 



I am working now with a platinum spiral heated by a current. I measure its 

 radiation by a pile and galvanometer, then suddenly 1 for an instant shunt it into the 

 bridge and find its resistance. I am getting very steady results with different battery 

 power. 



One of my students has attained great skill in finding specific heats ; and has 

 found that of best conducting copper to be slightly above that of bad, but to rise 

 more slowly with increase of temperature. 



I have asked Tyndall whether he couldn't induce the Shoeburyness people to 

 fire a few stone bullets at a stone wall and get a party with spectroscopes to examine 

 the resulting flash. I think comets might be thus elucidated. 



I sent a copy of my article to Lady Thomson last week. 



Yours T'. 



PS. Are you remembering poor Balfour and the Vortices ? 



PS. [ Written across the top of the first page of the letter^ Your sets of tenths 

 of a unit not o.k. I get different values when I use 100 and 1000 as the next sides 

 of the quadrilateral. For instance I find 1775 to 1000 and 179 to 100 for the same 

 pair of wires. 



In 1870 Tail began to communicate to the Royal Society his brief Notes 

 from the Physical Laboratory, the first set including J. W. Nichol's 3 experi- 



1 A marginal note by Thomson reads "March 28/71 Why suddenly? Rather keep it 

 always in the bridge under a constant El. M. F." 



1 J. W. Nichol, F.R.A.S., accompanied the Transit of Venus Expedition to the Hawaian 

 Islands, and published in the Proc. R. S. E. (Vol. ix, 1875) a graphic account of a visit to 

 Mauna Loa and Killauea, the remarkable volcanos with their lava lakes only 15 miles apart 

 but differing in level by 10,000 feet. He died young; and his mother founded in his memory 

 the Nichol Foundation in the Physical Laboratory of Edinburgh University. 



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