On a High Pressure Spark-Gap. 225 



the secondary of the Tesla inductor, and the discharge takes 

 place at D. In one experiment the balls in B were placed 

 0*3 cm. apart, and the distance between the terminals at D 

 was 7*0 cm. The pressure was normal. 



When the induction-coil was started only a luminous brush 

 of very thin sparks crossed at D. The distances remaining 

 the same in B and at D the pressure in B was raised to 

 75 cm. of mercury ; shortly before this pressure was reached, 

 a torrent of thick sparks passed at D, at the same time the 

 sparks in B became compact and exceedingly bright. On 

 removing the pressure the discharge returned to its former 

 condition. Some of the effects produced by the discharge, 

 though by no means of great length in air, were as follows: — 



A vulcanite tube 0*45 cm. thick in the wall was placed in 

 paraffin oil, a wire from one terminal being placed within it, 

 and outside it was placed a wire from the other terminal ; on 

 putting on the current the vulcanite was instantly perforated 

 by the discharge. 



Xext a piece of plate-glass 0*4 cm. thick was placed in the 

 oil, the terminals being placed face to face on either side of it. 



The instant the current was put on the glass was perforated 

 without being cracked. On replacing the thick glass by a 

 sheet of half the thickness, and repeating the experiment, a 

 nearly round hole (0*2 cm. in diameter) was made by the 

 discharge. 



In each case the conductor in contact with the glass was 

 pointed. A thick piece of plate-glass was also cut through 

 and divided into two parts by means of the discharge. The 

 sheet of glass was placed on a sheet of tinfoil in a flat vessel, 

 such as a photographic developing dish, and covered to the 

 depth of about 2 cm. with oil, the foil was connected to one 

 terminal, then on ruling a line with the other terminal on the 

 plate-glass as rapidly as the hand could move it, the glass 

 was found to be cut through under along the path of the 

 terminal. 



The discharge so produced, by the addition of the high 

 pressure spark-gap, is very effective when used to excite the 

 Rontgen effects in exhausted tubes and bulbs having no ter- 

 minals. A bulb (fig. 2) was placed in the line of discharge, 

 the distance between B and C being so arranged that but 

 little discharge passed before the introduction of the bulb ; 

 when the apparatus was in action a vivid green glow was 

 produced on the lower side of the bulb at A. In my early 

 experiments in 1#9G I used bulbs lof about 4 cm. in diameter. 

 The pictures were fairly good. I now find that in order to 

 obtain sharp definition the bulb should be small, about To cm. 



Phil. Mag. S. 6. Vol. 4. No. 20. Aug. 1902. Q 



