actuated by Resistance-temperature Variations. 1&3 



subject to the same errors ; the errors in the absolute values 

 of the energy delivered to the telephone circuit ma}- he 

 large. 



The most difficult observations to make were the steady 

 current ones. The current- voltage curves obtained with 

 increasing currents are very different from those got with 

 decreasing currents unless certain precautions are observed. 

 Usually a decrea sing-current curve lies above an increasing- 

 current curve. The difference between the curves was found 

 to depend greatly on the time allowed for a set of measure- 

 ments. If, with decreasing currents, the electromotive force 

 be held constant, the galvanometer reading slowly diminishes, 

 till in about half an hour it has fallen as a rule to the value 

 it would have at the same voltage with increasing currents. 

 On the other hand, if the voltage be raised suddenly from 

 zero to a fairly large value, the reading of the galvanometer 

 increases as time elapses, at first quickly and then slowly, 

 rising perhaps fifty per cent, in five minutes, and asympto- 

 tically approaching a limit. All these things are due to the 

 temperature at the contact lagging behind the changes of 

 current. This may be understood as follows : — At any stated 

 value of the steady current the mass of galena surrounding 

 the heated contact must be cooler when the current is being 

 increased than when it is being decreased — for the reason 

 that in the former case the conductor has just previously 

 been carrying smaller currents, in the latter case larger 

 currents, than the particular current considered. The rate 

 of loss of heat from the heated matter at the contact to its 

 surroundings is thus greater on the rising curve than on the 

 falling curve, and, in consequence, at corresponding points 

 on the two curves the resistance is higher in the former than 

 in the latter case. This temperature lag may be expected to 

 be more pronounced in detectors made wholly of substances 

 of low thermal conductivity than in detectors consisting of a 

 very thin film of oxide between metal electrodes. The phe- 

 nomenon is very prominent in this new galena detector 

 though it escaped observation in the iron-oxide detectors, and 

 therefore the former may be expected to work well as a 

 rectifier of alternating currents of moderate frequency. 



Once the slow movements of heat through the galena had 

 been observed the difficulties were overcome by allowing 

 time for thermal equilibrium to be attained before the final 

 galvanometer reading was taken. At places near the point 

 of inflexion of the steady current curve the time necessary 

 may be several minutes. 



Each setting of the two pieces of galena gives a different 



