1895.] Conductivities of Badly Conducting Substances. 269 



I could not detect any trace of conductivity during the formation 

 of the cloud. In this experiment the apparatus was sensitive 

 enough to detect the conductivity of a mixture of 1 c.c. of H^SO^ 

 in a litre of water. 



(2) The Calibration of a Bridge Wire. By E. H. Griffiths, 

 M.A., Sidney Sussex College. 



The ordinary methods of calibration involve changes in the 

 arrangement of the connections, during the operations, and thus 

 extreme care has to be observed to secure equality in the resis- 

 tance-contacts. It is chiefly on this account that the process 

 of calibration is invariably a lengthy and tedious business. In the 

 method I am about to describe the only contacts altered through- 

 out the observations are potential-contacts, and thus no attention 

 need be devoted to securing their equality. 



Let AB be the wire that is to be calibrated. (Its total resist- 

 ance should first be approximately determined in the ordinary 

 manner.) 



Let S^ and 8.^ be two cells of nearly equal electromotive 

 force, preferably two large storage cells. (Their absolute equality 

 is not essential.) 



In series with AB place some resistance R^ sufficient to 

 prevent any rapid fall in the E. M. F. of the cell S^. 



Make the total resistance of the circuit C^ approximately equal 

 to the total resistance of the circuit (7, . 



