ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 



131 



APPENDIX V. 



Table shoiving Values of Jive Standard Coils B.A. Units belonging to the 

 Indian Government as compared with Dr. 3fuirhead's Standard at his 

 Laboratory. By E. O. Walker, C.I.E., M.I.E.E., Late Superin- 

 tendent in the Government Telegraph Department in India. 



Standard used, No. 78, marked right at 15°-7 C, taken as correct. 

 This standard, tested April 27, 1893, against a No. 68 Glazebrook, gave a 



ratio — ^°!-, of 1-00015 at 16° C, and I-QOOIS at 15°-4 C 

 standard 



Temperature of water, 20° "2 C. 



Apparatus used, one metre bi'idge of platinum-iridium wire with a sup- 

 plementaiy coil at each end of 20,012 millimetres. Suspended coil galvano- 

 meter, resistance 15 ohms (Muirhead and Co.'s). Trough, 45 x 7|- x 5 inches ; 

 depth of water 2| inches ; quantity of water, 6^ gallons ; battery used, 

 1 Hellesen's Dry Cell, No. 3 ; E.M.F., 1-4 volt. 



The interest attaching to these tests lies especially in the fact that the 

 standard coils have been exposed to the climate of Calcutta for twenty- 

 four years. They were made, I understand, by Dr. A. Muii-head when in 

 Dr. Matthiessen's laboratory, under the supervision of the latter. 



In reducing the observations from 20°-2 to the temperatures given, it 

 has been assumed that all the coils have the same temperature coefficient. 



APPENDIX VI. 



On the Specific Resistance of Copper and of Silver. 

 By Rev. T. C. Fitzpatrick. 



As lately several observers have published the results of measure- 

 ments made on the specific resistance of copper, it may be worth while to 

 collect these results together in tabular form. 



The resistances of metals may be expressed in terms of equal weight or 

 of equal volume ; that is, as the resistance of a wire of the given material 

 such that one metre of it weighs one gramme, or as the resistance between 

 opposite faces of a cube of the material each face of which is one square 

 centimetre. I have pointed out that Matthiessen ' considered the first as 

 the most satisfactory mode of expressing resistances, and for these results 

 alone did he make all the actual experiments ; the results for specific 

 resistances were calculated from these with the help of specific gravity 

 values obtained in many cases from tables, and not determined directly 

 for the wires used. 



' B.A. Report, 1890, p. 129, 



K 2 



