upon the Electrical Resistance of Metals. 483 



that the two metallic bodies under comparison were, except 

 as regards the illumination, continuously under the influence 

 of identical conditions. The " damping" method did not per- 

 mit of two metallic bodies being simultaneously observed, and 

 was therefore employed to determine the resistance of one body 

 at frequent successive intervals, during which it was alter- 

 nately in the dark and under the influence of the luminous 

 rays. This method afforded no security against any possible 

 influence which alterations of temperature, and other changes, 

 might have upon observations which succeeded one another 

 after any considerable interval of time. Nevertheless the ma- 

 jority of the experiments have been carried out according to 

 this method, and far more favourable results have thereby been 

 obtained than with the Wheatstone bridge. The explanation 

 of this lies in the fact that the " damping " method is much 

 better adapted for working with electrical currents of very 

 weak intensity ; so that changes in the resistance, or other 

 influences, can only very slightly affect the currents which 

 are employed in the measurement. While in the first of these 

 methods a Leclanche cell was employed as the electromotor, 

 in the latter method the currents generated in the coils of 

 the galvanometer by the oscillations of the magnetic needle 

 were found to be sufficient ; and it appears that just this par- 

 ticular difference between the two methods had an important 

 influence on the results, and therewith also on the applicability 

 of the methods. 



The author then gives a general summary of the results 

 already obtained by other observers for selenium and tel- 

 lurium, and then proceeds to describe his own experiments. 



Experiments with the Wheatstone Bridge. 



In these, as well as in all the following experiments, the 

 connecting wires were always coated, and the junctions were 

 carefully soldered. Whenever a connection was made by 

 means of a mercury-cup, the wires dipping into it were amal- 

 gamated. The apparatus employed in the first of the methods 

 was of the form commonly known as the Wheatstone bridge. 

 The electromotor, a Leclanche cell, was connected directly 

 with a commutator, and a key, which usually kept the circuit 

 open, but closed it on being pressed down with the hand. A 

 second commutator was also employed for the purpose of 

 interchanging the two resistances whose ratio was required. 

 With the bridge-wire itself there was included a galvano- 

 meter, having a copper damper and a reflecting scale ; and 

 one end of this bridge wire was connected in the usual 

 manner with a metal slider, movable along the measuring- 



212 



