Inductive Capacity of Dielectrics. 



409 



forming the two bolometer arms corresponding to ah and ad 

 in fig. 2 ; and cb' and cd', together, constitutes a double coil 

 of wire, the two parts of which are of as nearly the same 

 resistance as it is possible to construct. 



The balancing arms of the bridge cb' and cd' consist of two 

 No. 22 insulated copper wires, each of about two ohms' resis- 

 tance, wound side by side on a brass tube. The lengths were 

 adjusted till the difference of resistance of the two wires was 

 less than one one-hundred-thousandth of an ohm, as indicated 

 by the Carey-Foster method. The bolometer arms of the 

 bridge were selected by trial of several pieces of No. 36 iron 

 wire till two were found that, when connected up as a Wheat- 

 stone bridge with the balancing arms already constructed, 

 gave no deflexion of the needle of a sensitive low-resistance 

 galvanometer when they were immersed in water at either 



Fig;. 3. 



20° or 100° C. The condensers c' and c'" are ordinary 

 adjustable air-condensers of about 12 and 16 centim. diameter 

 respectively. The condenser c" is shown in fig. 3. It con- 

 sists of two brass disks of about 3 - 2 centim. diameter imbedded 



