526 Dr. J. H. Vincent : Electrical Experiments 



rises. If when the lamp is alight the resistance is reduced 

 until we cross the line A'C, the arc lengthens and is ex- 

 tinguished. The volts on the lamp gradually rise during 

 this increase in length and the current tails. The ribbon 

 region bounded by the lines AB and A'C is comparable with 

 the necklace region bounded by the lines AB and AC in 

 fig. 2. In the case of the necklace above AB, each separate 

 arc keeps alight until the mercury columns come together; 

 below AC, each separate extinction occurs while the columns 

 are still separated. 



In the diagram fig. 15 which refers to a quartz tube 

 (54 cm., 4*2 mm., '37 mm.) when the pressure was 80 cm. 

 of mercury, it was found that the lines BA and CA' could 

 not be experimentally produced towards A and A' because 

 the lamp could not be lit on a voltage below 200. The 

 boundaries of the ribbon region could probably be better 

 determined by lighting the lamp on an electromotive force 

 of say 200 volts, and then lowering the voltage gradually by 

 the use of a suitable battery switch. 



The ribbon region on a diagram such as fig. 15 is always 

 bounded by two lines rising from left to right. The broken 

 lines ab, a'c on the diagram are the boundaries of the ribbon 

 region with 60 cm. pressure. With a fixed electromotive 

 force and under constant pressure the root mean square volts 

 across the lamp and the arc length decrease as the external 

 resistance increases, while the current and the mean electric 

 force in the arc are either constant or change very slightly 

 compared with the changes in the arc length and difference 

 of potential between the electrodes. A number of sets of 

 experiments in which the pressure and the electromotive 

 force were kept constant and the current, arc length, and 

 volts across the arc were measured as the resistance was 

 changed have been performed. Fig. 16 gives the result of 

 one such series. 



The current is generally nearly constant for each pressure 

 but rises with increase in resistance for higher pressures. 

 If the resistance is made the mean between the lowest and 

 highest on which the arc is stable for any given electro- 

 motive force and pressure, then the current falls with rise 

 of pressure, as below, for a fixed electromotive force of 

 400 volts. 



The volts per cm. of arc length are nearly constant for any 

 given pressure, but fall slightly for low pressures, and rise 

 slightly for high pressures, as the resistance is increased. 

 The volts per cm. of arc length corresponding to the mean 



