They were led to believe that intermissions of the current had a great deal to do with the production of the sensitive state, and accordingly they arranged for an air-gap in circuit with the machine and with the vacuum tube. § 51. They not only observed that such a gap caused the sensitive state, but that an increase in the length of the gap made the luminous column more sensitive. They increased the gap so much that the ramifications of the light could be seen. If an induction coil is employed as the secondary generator, a condenser should be coupled up in connection with it. The two in combination thereby produce the sensitive state, but upon cutting out the coil and charging the tubes from the condenser the sensitiveness can not be detected. Instead of the permanent air-gap, may be employed a rapid circuit interrupter, coupled up between a Holtz machine and a vacuum tube. The manner of coupling up is to place the interrupter in a shunt to the vacuum tube. Difficulty had been found in early experiments to obtain the sensitive state with those vacua which give striae. With a rapid circuit interrupter and an induction coil, the breaks occurring 240 per second, the luminous column was not only broken up into striae, but were acted upon by the approach of an outside conductor connected to earth. The sensitive state is not always made apparent by the appearance of attraction of the luminous light to the outside conductor. Sometimes the light seems to be repelled. These two phenomena may be caused in the same tube. This feature of the sensitive state constitutes the beginning of radiations of energy through the walls of a vacuum bulb, like X-rays. Some action or other in these cases takes place through the glass. They tried an experiment in which one of the electrodes of the vacuum tube was entirely on the outside. The electrical discharge was found to be sensitive, for the discharge was changed in its appearances by the presence of an outside conductor connected to earth. Another cause of the sensitive state was observed, namely, the brevity of the charge. This may be illustrated with a Leyden jar, which is known to give an almost practically instantaneous discharge. A single discharge from such a jar produced a flash of light which was in the sensitive state. The nomenclature by which the experimenters defined the cause of the phenomena is made up of the words: Re-distribution of electricity, and a relief of the external strain.