Mr. F. Jenkiu on the New Unit of Electrical Resistance. 483 



silver alloy, in a platinum-silver alloy, in a platinum-iridium alloy, 

 and in mercury. Two equal standards have been prepared in each 

 metal ; so that should time or accident cause a change in one or 

 more, this change will he detected by reference to the others. The 

 experiments and considerations which have led to the choice of the 

 above materials are fully given in the Report to the British Associa- 

 tion for 1864. The standards of solid metals are wires of from 0*5 

 millim. to 0*8 millim. diameter, and varying from one to two metres 

 in length, insulated with white silk wound round a long hollow bobbin, 

 and then saturated with solid paraffin. The long hollow form chosen 

 allows the coils rapidly to assume the temperature of any surround- 

 ing medium, and they can be plunged, without injury, into a bath of 

 water at the temperature at which they correctly express the standard. 

 The mercury standards consist of two glass tubes about three-quar- 

 ters of a metre in length. All these standards are equal to one 

 another at some temperature stated on each coil, and lying between 

 14°'5 and 16°*5 C. None of them, when correct, differ more than 

 0*03 per cent, from their value at 15°*5 C. 



Serious errors have occasionally been introduced into observations 

 by resistance at connexions between different parts of a voltaic circuit, 

 as perfect metallic contact at these points is often prevented by oxide 

 or dirt of some kind. Professor Thomson's method of inserting resist- 

 ances in the Wheatstone balance (differential measurer) has been 

 adopted for the standards, but in the use of the copies which have 

 been issued it has been thought that sufficient accuracy would be 

 attained by the use of amalgamated mercury connexions. 



In the standards themselves permanence is the one paramount 

 quality to be aimed at ; but in copies for practical use a material 

 which changes little in resistance with change of temperature is 

 very desirable, as otherwise much time is lost in waiting till coils 

 have cooled after the passage of a current ; moreover large corrections 

 have otherwise to be employed when the coils are used at various 

 temperatures ; and these temperatures are frequently not known with 

 perfect accuracy. German silver, a suitable material in this respect, 

 and much used hitherto, has been found to alter in resistance, in 

 some cases, without any known cause but the lapse of time, since 

 the change has been- observed where the wires were carefully protected 

 against mechanical or chemical injury. A platinum-silver alloy has 

 been preferred by the Committee to German silver for the copies 

 which have been made of the standard. These have been adjusted 

 by Dr. Matthiessen so as to be correct at some temperature not dif- 

 fering more than 1° from 15°*5 C. The resistance of platinum-silver 

 changes about - 031 per cent, for each degree Centigrade within the 

 limits of 5° above and below this temperature ; this change is even 

 less than that of German silver. The new material seems also likely 

 to be very permanent, as it is little affected by annealing. The form 

 of the copies is the same as that of the standard, with the exception 

 of the terminals, which are simple copper rods ending in an amalga- 

 mated surface. Twenty copies have been distributed gratis, and 

 notices issued that others can be procured from the Committee 



