168 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
electricity of high tension, and interposing a small air-spark in the path of the current 
from one of the terminals of the machine to the tube. The following plan will 
sufficiently describe the apparatus used. So soon as the air-spark is made to intervene 
7o.Mae/u'?ie To Machzzie 
(see Plate 15, fig. 1), the discharge in the tube becomes sensitive ; and this sensitiveness 
may in general be increased by increasing the length of the air-spark, until the 
discharge becomes visibly intermittent, so as no longer to appear to the eye as a 
steady continuous discharge. Although this is by no means the only way in which 
the sensitive discharge can be produced, it is the one which is the most generally 
convenient for the purposes of experiment; and it may on that account be regarded 
as the typical mode of production. That the discharge within the tube should be 
intermittent when the supply of electricity is rendered intermittent by the air-spark 
is so obvious as not to need discussion, and though we cannot positively assert (without 
experimental evidence of the fact) that the pulses of free electricity of which the dis¬ 
charge is composed pass into the tube instantaneously, it is difficult to imagine any 
circumstances under which we should less expect the individual discharges to have a 
prolonged or continuous character than when, as in the present case, they are due to 
charges of free electricity that come suddenly to the terminal of the tube after leaping 
in the form of a disruptive discharge across the air-spark interval. 
Another method of producing a discharge which shows indications of sensitiveness 
is by using an induction coil in connexion with a large condenser, such as that 
described in a paper “On Stratified Discharges, V.” (Proceedings of the Royal 
Society, xxvii., p. 60). If the discharge be allowed to pass through the tube while the 
