508 



Mr. J. S. G. Thomas 



on 



to within 0'2 per cent. The drop of potential along AB 

 could be measured by means of the Weston voltmeter or! 

 resistance about 1000 ohms, which could be inserted for the 

 purpose by means of the key K. The resistances of the wires 



Fi<r. 3. 



AB and BC when heated by the currents employed were of 

 the order 0*5 ohm. It is evident that to 1 part in 2000, the 

 whole of the current in the bridge when balanced, passes 

 through the arms AB and BO. A bridge connected and em- 

 ployed in this manner may be described as a constant-current 

 bridge, possessing maximum sensitiveness in accordance with 

 Maxwell's rule, and with the maximum generation of heat in 

 the sensitive branch. The bridge employed in King's expe- 

 riments was of the constant-temperature t\pe, the balance of 

 the bridge after disturbance due to the cooling effect of the 

 current of fluid being restored by the passage of additional 

 current through the wire. 



Fur. 4. 



ft ft 



TO 

 A 



The general mode of employing the anemometer in the 

 present series of experiments is shown in fig. 4. The air or 



