TROWBRIDGE. 



DISCHARGES OF ELECTRICITY. 



459 



D', and is marked in the direction toward A by a white beam which 

 produces hardly a perceptible fluorescence. The latter beam does not 

 come from the metallic surface of the cathode, but seems to come 

 from the gas in the region DD'. At comparatively high exhaustions 

 this latter portion of the cathode beam ceases to ionize the gas and 

 the current ceases ; the potential between A and D rises to the full 

 potential of the battery — indicating an open circuit. When, how- 

 ever, D' is made the cathode, the current is immediately re-estab- 

 lished and the cathode beam from D' ionizes the gas between D' and 

 A. The tube acts as a rectifier ; for when D is made the anode and A 

 the cathode, a current 

 passes ; on reversal of 

 the current, when at the 

 same exhaustion, no cur- 

 rent passes in the op- 

 posite direction. 



It is interesting to 

 observe the effect of + 

 a transverse magnetic 

 field on the discharge 

 in this tube when A is 

 made a cathode and D 

 an anode, and striae ap- 

 pear in the portion DD'. 

 The magnetic field 

 placed near A diverts the 

 portion DD'. While this 



Figure 7. 



cathode beam and striae advance in the 

 field is still on, another transverse mag- 

 netic field placed near D' diverts the striae independently of the action 

 of the field at A. This indicates the well known fall of potential 

 from striae to striae. 



The rectification observed under proper conditions in the tube ( Fig- 

 ure 6 ) suggests other forms of tubes by which rectification can be pro- 

 duced. Even with a straight cylindrical tube the current can be stopped 

 at high exhaustions by touching the outside of the tube with the fin- 

 ger, thus diverting the cathode beam by electrostatic action ; while it 

 readily passes when the current is reversed. The phenomenon of rec- 

 tification is shown in a practical way in the U-shaped tube represented 

 in Figure 7. It is provided with two anodes, A and A', and two cath- 

 odes, D and LV The cathodes have orifices at their centres. The 

 two anodes are connected together, and the two cathodes — the tube 

 forming a multiple circuit. A transverse magnetic field can be so 

 placed near one cathode that no current will pass in the branch of the 



