ST. JOHN. 



WAVE LENGTHS OP ELECTRICITY. 



233 



Fig. 7. 



net, of which Wo is a portion. If in the quadrilateral A B C D the 

 sides are of equal resistance, and if then a current be led in at B and 

 out at D, or vice versa, no difference of potential will be produced 

 between A and C ; in the same way, a current can be led in at A and 

 out at C without producing 

 any potential difference be- 

 tween B and D. The two 

 currents can traverse the rec- 

 tangle simultaneously and ex- 

 ert no effect upon each other 

 arising from difference of po- 

 tential. If, after the bolom- 

 eter branch and the bridge 

 are both balanced, a current, 

 alternating or direct, is sent 

 through the bolometer branch 

 W2 from B to D, none of the 

 current will pass through the 

 galvanometer ; but the resist- 

 ance of the branch Wg will be increased by the evolution of heat, 

 and the bridge thrown out of balance, as indicated by the deflection of 

 the galvanometer. 



The resistances W3 and W4 were coils of fine German silver wire 

 double wound on wooden spools, and of three ohms' resistance each. 

 The quadrilaterals W^ and W^ were of iron wire, radius 0.035 mm., 

 and each side had a resistance of three ohms. The arms of the 

 bolometer branch Wg were balanced before use by means of the slid- 

 ing mercury contact D, so that no deflection was produced in the gal- 

 vanometer when a steady current was passed from B to D. The 

 sliding mercury contact consisted of 20 cm. of No. 18 German silver 

 wire, L M, and a sliding block of brass, D, which contained in its 

 upper surface a small cup-shaped cavity filled rounding full with 

 mercury. The German silver wire was amalgamated to insure a good 

 contact. The contact G was of similar construction. The connections 

 were of No. 18 copper wire, whose resistance was negligible in com- 

 parison with the bridge arms. 



The adjustment of the bolometer branch, once made, remained 

 constant through the series of observations, but the bridge adjustment 

 by means of the sliding contact G had to be made frequently. To 

 supply the bridge current a Daniell cell was used with a resistance 

 of 5 to 30 ohms in circuit. 



