ON THE B.A. UNITS OF ELECTRICAL RESISTANCE. 13 



all the theorems which directly depend on the definition of proportion, indi- 

 cating that the demonstrations are to be adapted to the complete or to the 

 partial theory according as the one or other has been studied. Alter these 

 follow the usual standard theorems on Similar Pigures, &c., on which it is un- 

 necessary for the Committee to oifer any comment. 



The Association for the Improvement of Geometrical Teaching has not yet 

 published any Syllabus of Solid Geometry. Should the present Syllabus of 

 Plane Geometry be successful in leading to the establishment of a standard 

 sequence of propositions in that subject, it is to be hoped that the Association 

 will continue its labours in the field of Solid Geometry, where the Committee 

 believe they are equally needed. 



Results of a Comparison of the British- Association Units of Electrical 

 Resistance. By G. Chrystal and S. A. Satjnder*. 



[A communication ordered by the General Committee to be printed in extenso 



among the Reports.] 



BifficuJtii's encountered. — The difficulties of the kind of measurement we 

 had to make are confined almost entirely to the temperature determinations. 

 Were it not for these a much higher degree of accuracy could be attained ; 

 for while resistances comparable with the B.A. unit can be measured with- 

 out difficulty to the 100,000th part, it is very difficult to determine the tempe- 

 rature of a wire imbedded in paraffin, as are the wires of the standards, 

 nearer than the one tenth of a degree Centigrade, an error to which extent 

 entails in some of the coils an error of -03 per cent, of resistance. 



A mere comparison of the coils at the temperatures given on page 483 of 

 the B.A. Report on Electrical Standards (1867)t would hardly have been satis- 

 factory, since it would have given no check on the accuracy of the observa- 

 tions and atforded no information as to the temperature value of a variation 

 in resistance, and conversely. 



Object aimed at. — The object aimed at in the experiments was to get the 

 diff'erences between the resistances of the several coils at some standard 

 temperature, and also the coefficients of variation of resistance with tempe- 

 rature in the neighbourhood of the standard temperature. 



That it is inadmissible to apply to any given coil the variation-coefficient 

 for its supposed material, as found by Matthiessen and others from experi- 

 ments on naked wires, is abundantly evident. This appears very strikingly 

 in the case of coils Nos. 2 and 3 (A and B in our subsequent numbering) ; and 

 an examination of the results of Lenz, Arndtsen, Siemens, and others for 

 platinum shows that within certain limits its behaviour is very uncertain. 

 This arises no doubt from the presence of more or less iridium or other 

 platinoids, a small admixture of which, without altering the value of platinum 

 commerciall}', aftects its electric resistance very considerably. 



* In the spring of last year a series of experiments was made by one of the authors 

 (G. Chrystal) with a view of comparing the different resistance-coils of the set of British- 

 Association units formerly deposited at Kew Observatory and now in the Cavendish 

 Laboratory at Cambridge. In the month of October a final set of experiments was made, 

 which was the work of both of us, sometimes working together and sometimes separately. 



t Or Eeprint, p. 146. 



