X. STANDARDS. 345 



receiving instrument, or between the latter and the earth in order 

 to render the signals more distinct. 



Warden, Muirhead, and Clark. 



15O4f. Standard Condenser of 5 microfarads capacity sub- 

 divided int& tenths. Warden, Muirhead, and Clark. 



1504g. Standard Condenser of 5 microfarads capacity. 



Warden, Muirhead, and Clark. 



15O4g. Sine Inductor. Apparatus for producing an induc- 

 tion current by the rotation of a magnetic disc for determining 

 resistance. L. Waibler, Darmstadt. 



15O7. Conductivity Apparatus, for comparing the conduc- 

 tivity of wires of copper, or any other metal or alloy, with 100 

 inches of pure copper weighing 100 grains, 2CO inches weighing 

 100 grains, or 300 inches weighing 100 grains. The conductivity 

 is determined by resistance, length, and weight. 



Elliott Brothers. 



X. STANDARDS OF COMPARISON FOR MEASURE- 

 MENT OF ELECTRICAL MAGNITUDES. 



1505. Weber's Resistance Coils, No. 3 and No. 5, certified 

 to represent the following resistances : 



No. 3 = 60717 x 10 5 Weber's absolute units. 



No. 5=59440 xlO 5 



J. Bosscha, Professor, Royal Polytechnic School, Delft. 



Comparisons of these coils were published by Dr. Schroder van der Kolk 

 in Poggendorff's Annalen, vol. 110, p. 465. 



The coil No. 3 was used in the first determination of the electro-motive 

 force of a Daniell's cell in absolute measure, executed in 1857 by Dr.Bosscha. 

 This electro-motive force was found to be 1026 x lo 8 Weber's absolute units. 

 Applying Sir William Thomson's theorem (Phil. Mag., Series IV., vol. ii. 

 p. 557). the mechanical equivalent of heat derived from this result is 432' 1 as 

 compared with MM. Favre and Silbermaun's determination of the heat gene- 

 rated by the action of zinc on sulphate of copper ; 419 '5 as compared with 

 Mr. Joule's experiments on the heating power of a Daniell's cell (Phil. Mag., 

 Series IV., vol. iii.), discussed in Poggendorff's Annalen, vol. 103 ; 437*3 as 

 compared with similar experiments of Professor Lcnz (Pogg. Ann., vol. 59, 

 p. 203), discussed in Poggendorff's Annalen, vol. 108, p, 162 ; and 421 ! as 

 compared with similar experiments of Mr. Joule, related in " Memoirs of the 

 Lit. and Phil. Soc. at Manchester," vol. vii. p. 94, and discussed in Poggen- 

 dorff's Annalen, vol. 108, p. 168. 



1506. Siemens' Resistance-Unit. 



Siemens and Halske, Berlin. 



This is a wire resistance equivalent to the resistance offered by a prism of 

 pure mercury one square millimetre in section and one metre long at centi- 

 grade, which is the basis of the Siemens' standard unit. 



