160 Mr. F. Jenkin Report on the New Unit [April 6, 



that, if hereafter improved methods of determination in absolute measure 

 are discovered or better experiments made, the standard need not be 

 changed, but a small coefficient of correction applied in those cases in which 

 it is necessary to convert the B. A. measure into absolute measure. Every 

 unit in popular use has a distinctive name ; we say feet or grains, not units 

 of length or units of weight ; and it is in this way only that ambiguity can 

 be avoided. There are many absolute measures, according as the foot and 

 grain, the millimetre and milligramme, the metre and gramme, &c. are 

 used as the basis of the system. Another chance of error arises from the 

 possibility of a mistake in the decimal multiple used as standard. For 

 all these reasons, as well as for convenience of expression, the writer 

 would be glad if Mr. Clark's proposal were adopted and the unit called an 

 Ohmad. 



Experiments have been made for the Committee by Dr. Matthiessen, to 

 determine how far the permanency of material standards may be relied 

 on, and under what conditions wires unaltered in dimension, in chemical 

 composition, or in temperature change their resistance. Dr. Matthiessen 

 has established that in some metals a partial annealing, diminishing their 

 resistance, does take place, apparently due to age only. Other metals 

 exhibit no alteration of this kind ; and no permanent change due to the 

 passage of voltaic currents has been detected in any wires of any metal 

 a conclusion contrary to a belief which has very generally prevailed. 



The standard obtained has been expressed in platinum, in a gold-silver 

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

 cury. 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 be 

 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 Association 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 siik 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 

 surrounding 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-quarters 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 re- 

 sistance 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 resistances in the Wheat- 

 stone balance (differential measurer) has been adopted for the standards, but 



