EMISSION AND 'TRANSMISSION OF KONTGEN RAYS. 127 



was largely diminished by using the least current in the primary circuit of the coil, 

 which would just cause the discharge to pass in the tube. 



The vacuum could, however, be kept very fairly constant by keeping the apparatus 

 joined to the charcoal tube, and disposing and maintaining the level of the outside 

 liquid air until a convenient working pressure was arrived at. 



Another and excellent plan, which was latterly always adopted, is to saturate the 

 charcoal with gas at the pressure desired, and keep it entirely immersed in the liquid 

 air. By this means the pressure can be maintained constant for hours together, no 

 matter how intense the discharge. 



In a comparison of the Rontgen rays from different metals it is, of course, essential 

 to keep the current in the primary circuit of the coil constant, and an ordinary 

 * hammer-break interrupter cannot be relied upon to do this. A mercury turbine 

 interrupter working in spirit was used. It proved reliable if periodically cleaned, and 

 worked more steadily at high than at low speeds. Any small variations indicated 

 by the ammeter in the primary circuit could be followed and corrected for by an 

 adjustable resistance. An increase in the current through the primary, besides 

 increasing the intensity of the Rontgen rays, has also the effect of augmenting the 

 length of the equivalent spark gap of the tube. 



The aluminium window was 0'0065 cm. thick, and 2 cms. in diameter, and 

 perceptibly sagged under the outside pressure. It was gripped between two stout 

 brass rings screwed together. One of the rings was slipped on to the glass tube B, 

 and the joint was completed with sealing-wax. 



The ionisation vessel consisted of a flat cylinder about 9 cms. diameter and 4 cms. 

 deep, with its ends covered with aluminium leaf. The middle of the front face was 

 about 4 cms. from the aluminium window. A central insulated aluminium ring, over 

 which was stretched aluminium leaf, was mounted with its plane parallel to the ends 

 of the cylinder. It was insulated by sulphur in an earthed guard tube which was 

 mounted in an ebonite plug let into the side of the cylinder. The connection to the gold 

 leaf of a Wilson tilted electroscope E (fig. 2) was shielded by earthed tubes. The outside 

 of the ionisation chamber was raised by means of a battery of small accumulators, with 

 the negative pole earthed, to a potential (200 volts) sufficient to give a saturation 

 current for any type of Rontgeii ray. A lead enclosure shielded the electroscope, and 

 another surrounded the discharge tube. 



Measurements. 



The very great range in sensitiveness that a tilted electroscope provides was a great 

 convenience in the present work, owing to the large variations in intensity of the rays 

 dealt with. A potential divider, P, giving a control of a fraction of a volt, was used 

 to furnish a fine adjustment on the potential of the charged plate of the electroscope, 

 which ordinarily was raised to something in the neighbourhood of 200 volts. 



