274 THE ELECTRIC TELEGRAPH. 



thiessen's experiments, undertaken for the Unit Committee 

 of the British Association, show that annealed German- 

 silver increased its conducting power in the course of a 

 year at the rate of nearly 0*2 per cent. ; some specimens 

 of gold, silver, and copper, both annealed and hard-drawn, 

 also altered their conducting powers. The change in the 

 conducting power of hard-drawn silver was the most con- 

 siderable, the two specimens experimented upon having been 

 found to have increased at the rate of over 3 '9 and 2*8 per 

 cent, respectively. 



That some of the specimens of metals experimented with 

 should have shown no change is no argument that they 

 would not do so if allowed to get older ; and errors of 

 observation may sometimes, especially in measuring such 

 minute differences, as well be taken to account for agree- 

 ment as for disagreement. At present the question must 

 be considered an open one, whether metals do change 

 their conducting powers by age ; and if so, if the change 

 in the same wire is always in the same sense ; and if so, 

 what becomes of the conducting power at last, has still to 

 be determined. 



Another vexed question to be set at rest is, whether the 

 passing of electric currents through a wire is able to alter 

 its conducting power ? 



Professor Kirchoff says that the conducting power of any 

 wire, at a given temperature, certainly undergoes changes if 

 electric currents are transmitted through it and it is exposed 

 to fluctuations of temperature. Schroder van der Kolk also 

 says that the conducting power of a copper wire undergoes 

 a change whenever weak currents are allowed to pass 

 through it. 



If this were the case, of what use would be our resistance- 

 scales? Dr. Matthiessen has happily found this to be a 

 fallacy. He allowed a current from two Bunsen's cells 

 to pass through a series of wires of different metals for 

 six days, at the end of which no change in their conducting 

 powers had taken place a result which some experiments 

 of our own, undertaken in the winter of 1862-3, in Germany, 



