ON STANDARDS OF ELECTRICAL RESISTANCE. 475 



Last year preliminary experiments of this kind had been made by Dr. 

 Joule, and the agreement which he then reported between his mechani- 

 cal equivalent obtained by frictional experiments and that obtained by the 

 electrical method was so close as to lead to a suspicion that it was partly 

 fortuitous. 



The experiments, which have this year been conducted vrith every pos- 

 sible care, give 783 as the value derived from the B.A. standard of resist- 

 ance, while 772 is the well-known number derived from fi'iction. 



The details of the experiments are contained in an Appendix which accom- 

 panies this Report. Dr. Joule states his opinion that the electrical method 

 has been carried oiit with greater accuracy than the frictional method, 

 assuming the E.A. standard to be an exact decimal multiple of the absolute 

 unit. The following extract from Dr. Joule's Report T\'ill show the labo- 

 rious nature of the experiments. Ho says, " The last and most perfect series 

 of experiments comprise tliirty for the thermal effect of currents in the 

 spiral, thirty for the effect of radiation &c., and thirty for the horizontal 

 intensity pf the earth's magnetism." Di-. Joule expresses himself willing 

 to make a new determination by friction. Meanwhile the experiments 

 already completed remove aU fear of any serious error, cither in the number 

 hitherto used as " Joule's equivalent," or in the B.A. standard, a fear which 

 hitherto, remembering the very discrepant results obtained by others, has 

 been very naturally entertained even by the Sub-committee, from whose 

 experiments the standard was constructed. 



In connexion with the measurement of resistances, Mr. C. W. Siemens 

 has invented a simple and excellent contrivance, by which the measurement 

 of resistances can be made by persons wholly unaccustomed to electrical 

 experiments. They have only, after the necessary connexions arc made, to 

 turn a screw tUl a needle stands opposite a fiducial mark, when the resist- 

 ance required may be read directly on a scale with considerable accuracy. 

 Mr. Siemens proposes to apply this invention to pyrometers, where the 

 resistance read will indicate the temperature, and the only electrical con- 

 nexions required will be joining of the battery wires to two terminals. 

 Other applications of this invention wUl doubtless arise, and extend the 

 practical application of electrical measiirements. A full description of the 

 instrument is contained in the Appendix. Mr. Siemens reports very favour- 

 ably of this instrument, which possesses considerable advantage in cheap- 

 ness and portability. Mr. Siemens has constructed the instrument, and made 

 the experiments entirely at his own expense. 



An instrument similar in object, and suggested by the above, is also 

 described by Sir. Jenkin in an Appendix. 



Mr. Hockin has tested the constancy of the standard resistance-units, with 

 satisfactory results, except in the case of one mercury tube. The exact 

 results of Mr. Hoekin's comparisons are appended. He suggests that lead- 

 glass was used for the mercury tube, and that the glass may consequently 

 have been injured by the nitric acid used to clean it. 



Mr. Hockin lias also made interesting experiments on the construction of 

 large resistances by the use of selenium. He finds that resistances of one 

 million units and upwards can be made of this material, and that these 

 artificial resistances maintain a sensibly constant resistance at high tempera- 

 tures, such as 100° C. It is hoped that these very high artificial resistances 

 wiU be found useful in practice and much superior to those hitherto con- 

 structed of gutta pcrcha, or other insulators, which were of comparatively 

 little use in accurate work, owing to absorption, change of resistance with 



