1898.] Fall of Potential at the Electrodes, etc. 21 



By pressing the key K, the wire W and the gold-leaf G can 

 be simultaneously charged to any required potential by means of 

 the wire leading to a battery of cells fi. 



The guard-tubes H and J are also kept at the same potential 

 by the wires N and M, and the same battery maintains the plates 

 P and P' at a constant difference of potential. 



The gold-leaf method here employed for detecting the changes 

 of potential of the wire, although in the form used it does not 

 detect as small differences as a quadrant electrometer, still has an 

 advantage in that owing to the small capacity of the system it 

 takes up the desired potential of a place much more rapidly, which 

 is a great convenience when working with the small conductivities 

 here concerned. 



By making the distance between the plate D and the gold- 

 leaf G such that for the voltage to be used the leaf is almost on 

 the point of being drawn to the plate, the indications are suffi- 

 ciently sensitive for the purpose in hand. 



The following is the procedure for finding the potential of any 

 place between the plates during conduction. 



The investigating wire W being placed in the desired position, 

 the wires and M (fig. 3) are connected to the battery /3 to a 

 point a which has a potential estimated to be that of the place in 

 question. 



The key K is now opened, leaving the wire W and the leaf G 

 insulated and charged to a known potential. 



The rays are now allowed to pass through the aluminium 

 window A and to traverse the space between the plates, and 

 according as the wire was charged to a lower or higher potential 

 than that of the required point the gold-leaf shows an increase or 

 decrease in deflection, the wire gaining a charge from or losing it 

 to the gas. 



The wires and M are connected successively to different 

 points of the battery until for two adjacent cells the gold-leaf 

 indicates an increase in charge for one and a decrease for the other, 

 when the rays are turned on. 



By noting for the steady state in these two cases the number 

 of divisions of displacement on the microscope scale, the desired 

 potential is obtained by interpolation. 



The presence of the guard-tubes H and J was found to be 

 important, since, being kept at the same potential as the insulated 

 wire W, it is impossible for this wire to gain or lose a charge by 

 gas conduction at any point except that between the plates whose 

 potential is desired. 



Without the guard-tubes a small leakage is liable to occur 

 from those parts of the wire which are outside of the plates, 

 although the direct rays do not impinge upon any of the lines of 



