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Mr Searle, A method of determining the 
A method of determining th e ther mal conductivity of Indian libber. 
By G. F. C. Searle, M.A., F.R.S., University Lecturer in Experi- 
mental Physics. 
[Read 28 January 1907.] 
§ 1. Though the principles of thermal conduction are of great 
importance in the study of heat, the direct determination of 
thermal conductivity presents considerable difficulties in the 
general case and thus there are but few methods of determining 
thermal conductivity suitable as exercises for elementary students. 
The difficulties of the subject can, however, be considerably 
reduced by using solids of very high or of very low conductivity, 
such as copper, on the one hand, or indiarubber on the other. The 
determination of the thermal conductivity of copper has been one 
of the regular class experiments at the Cavendish Laboratory for 
some years. I now describe a students method for determining 
the thermal conductivity of indiarubber, which was originally 
devised by Mr C. T. R. Wilson and myself to furnish a question 
in the examination in Practical Physics in Part I of the Natural 
Sciences Tripos, 1906. 
The method is, in principle, applicable to any substance of very 
low thermal conductivity, and only differs from Peclet’s original 
method by the substitution of a tube for a plate. But the 
flexibility of an indiarubber tube allows the use of very simple 
apparatus. 
The linear measurements which must be made upon the india- 
rubber are not capable of any great accuracy, and thus the method 
must be regarded rather as furnishing a useful illustration of the 
theory of conduction than as a means of obtaining an accurate 
value for the conductivity. Yet the results obtained by students, 
who use the method for the first time, usually agree among 
themselves to within 5 per cent. 
Fig. l. 
