ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
211 
symmetry of the electric condition of the terminals the arrangement is equivalent to 
an air-spark at both ends, and that the positive and negative pulses are approximately 
equal in quantity, and rush simultaneously along the tube so as to meet about the 
centre. The parts near the centre will thus be continually in a state of no electric 
tension, and hence there can be no relief-effect and no sensitiveness. The pulse at 
either terminal is of the opposite sign to the pulse at the other terminal, and hence to 
the pulses that strive to leave the tinfoil upon the tube near that terminal. Hence 
the connexion of a piece of tinfoil with the more distant terminal will naturally 
produce the relief-effect, though in a stronger and more decided form. And, moreover, 
the relief-effects in the two portions of the tube are of opposite characters, showing 
conclusively that the corresponding electrical pulses within the tube are of opposite 
signs. 
A slight variation of the experiment tends to confirm this view. If we join one of 
the terminals to earth, so that these discharges cannot produce electrical tension or 
elevation of potential there, we find at once that the neutral zone disappears, that the 
luminous discharge presents all the characteristics of an ordinary sensitive discharge 
with the air-spark at the terminal opposite to that which is to earth, and that the 
sensitiveness decreases in the usual manner throughout the whole length of the tube 
as we proceed from the air-spark terminal towards the other end of the tube. And 
if instead of connecting the terminal to earth we connect it to a condenser of small 
capacity whose other surface is to earth, so as to diminish, without completely sup¬ 
pressing, the variations of tension, we find an effect of an intermediate character 
produced. The neutral zone is displaced towards the terminal which is connected 
with the condenser, and the sensitiveness is found to decrease as we pass from either 
terminal towards the neutral zone. 
A striking confirmation of the correctness of the interpretation here given of these 
phenomena is obtained by placing a broad piece of tinfoil round the tube midway 
between its extremities, and putting a telephone in circuit between the tinfoil and the 
earth. When discharges from a small coil (such as above described) are sent through 
the tube, the sound heard within the telephone will be faint; but it instantly springs 
into loudness upon either terminal of the tube being touched with the finger. The 
reason is obvious. By touching one of the terminals, the discharge becomes one in 
which the same kind of electricity passes throughout the tube; and the wants of the 
tinfoil are all of one kind, and are therefore additive. But when both discharges 
enter the tube, the wants of opposite ends of the tinfoil are of different signs, and it is 
only the balance which has to come through the telephone circuit. 
Still more striking effects in illustration of the subject were obtained by the use of 
a larger coil with a tube moderately exhausted, and a break working at a somewhat 
rapid rate. The luminous discharge presented the ordinary characteristics of a coil- 
discharge as we have just described them. When a ring of tinfoil, somewhat less than 
an inch in breadth, was placed round the tube not far from one end and connected 
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