H26 Prof. Skinner on the Drop of Potential 



drop " increase with increased currents, the latter more rapidly 

 than the former. Contrary to the expectation expressed 

 above, it is observed that while the drop in the cathode dark 

 space decreases, that at the cathode increases. The observed 

 "" immediate drop " is, however, the drop between cathode 

 and gas plus the drop in a certain portion of the gas which 

 cannot be eliminated. In fact, the observations of Graham 

 (I. c. p. 173) show that with increasing current the gradient 

 in the immediate vicinity of the cathode increases, while 

 farther away it decreases. It was noticed in this connexion 

 that when once the zero gradient in the cathode dark space 

 was ushered in by a large current, it held then also with 

 smaller currents, indicating that the gradient is influenced by 

 the condition of the gas. At the higher pressures (above 

 2'h mm.) it could not be made to vanish entirely. 



It may be noted here that the " immediate drop " at the 

 cathode for the same current decreases rapidly with increas- 

 ing gas-pressure, as might be expected if an increased number 

 of molecules in unit volume of the gas increase the facility 

 for discharging the ions. 



While carrying out these observations, it was found that 

 the gas underwent during use a change of somewhat peculiar 

 nature. So long as the pressure was not varied by drawing 

 off part of the gas (or adding new) consistent results were 

 ■obtained with different fillings of gas, but the presence of old 

 gas at the new pressure always lowered, more or less, the 

 value of the cathode drop. 



The drop between cathode and the negative glow being 

 under certain conditions a [negative] definite quantity, it was 

 carefully studied with reference to current and gas- pressure. 

 For each new gas-pressure entirely fresh gas was used, and the 

 gradient in the cathode dark space made to approach a zero 

 value (if possible) by passing first a sufficiently large current. 

 Observations were then immediately taken, and, if at the 

 close of a series no " time-change " was observable, the 

 results were considered good. They are given, for a steel 

 •cathode, in Table V., and plotted in fig. 3 (p. 628) . In the figure 

 each line represents for the indicated gas-pressure the relation 

 between drop in volts and the current in milliamperes. These 

 show that with constant pressure the drop at the cathode is a 

 linear function of the current. As the pressure decreases the 

 inclination of this line to the axis of abscissas increases. The 

 line appears to start from a horizontal position at high gas- 

 pressure, and as this is decreased without limit, gradually 

 rotates about a fixed point in the second quadrant as centre 

 toward a vertical position. Within experimental variations 



