1 72 Mr. R. Sabine on a New Determination of 



The resistances (W 2 ) were produced by means of combinations. 



For tube 3, resistances of 10, 12, and 13 parallel; for tube 

 5, resistances of 3, 10, 12, and 13 parallel; and for tube 11, 

 resistances of 10, 14, and 15 one after the other. 



Such resistances as I have described are, however, incommo- 

 dious on account of their length and the difficulty of keeping 

 them, without special apparatus, at a constant temperature 

 during the measurements. Dr. Siemens's method of copying 

 them in the form of glass spirals, however, removes all difficulty 

 on this head, particularly as it is evident they may be copied 

 with great exactness. The only modification which it was thought 

 advisable to introduce, was in double- winding the spirals to pre- 

 vent induction currents, and in making the cups or pieces of 

 wide glass tube at their ends separate, to be cemented on or fixed 

 with thick india-rubber rings, instead of being fused on as was 

 done previously. The spirals are easier made, and are not so 

 easily broken. Copies of the normals have been made approxi- 

 mating to 0*1, 0-2, 0*5, 1*0, and so on, up to 10 metres' resistance, 

 by direct comparison with combinations of the normal tubes ; and 

 beyond 10 and up to 50 metres, two or three spirals of nearly 

 equal resistances were first compared with each other, and then 

 their combined (parallel) resistance with a combination of normals 

 of about the same value. 



By grinding off the ends, they can, of course, be made exact 

 multiples of the unit. 



The subsequent process of copying these spirals in metal wire 

 for resistance-scales is obvious. 



Following this method, every electrician may inexpensively 

 and with little trouble make himself a standard measure. It is 

 of course needless to determine the capacities of ten tubes, as was 

 done in this reproduction, the only purpose in view in the mul- 

 tiplicity being to ascertain the maximum disagreement between 

 the tubes when compared as resistance-measures. This maximum 

 has been shown to be within the amount of errors of observation 

 in the comparison. The mercury unit has therefore been pro- 

 duced in Dr. Siemens' s laboratory twenty-one times — six times 

 in the first determination, five times in the second, and ten times 

 in the present. And, allowing for the unfortunate misrepresen- 

 tation of the measure by individual errors of the measuring- 

 apparatus used in the comparison of the first tubes, the agree- 

 ment between them all is greater than could be guaranteed 

 between any two single electrical measurements with different 

 measuring-apparatus. 



From the foregoing results it follows that, by the method of 

 direct production proposed by Dr. Siemens, much greater exact- 

 ness has been attained than by means of any of the other methods 

 of determination or copying. 



