Rtintgen rays and the radiation from radio-active substances. 357 



The ionization vessel is a vertical brass cylinder 12 cm. in 

 diameter and 19 cm. in length. A brass rod 2 mm. thick and 

 81 cm. long passes through a small hole in the centre of the top 

 of the cylinder. It is fixed in the cylinder and insulated from it 

 by a plug of sulphur. 



The bottom of the cylinder has a large hole 6 cm. in diameter, 

 which is covered with a thin leaf of aluminium. 



The electrometer is furnished with a small adjustable con- 

 denser G, consisting of two circular discs 5 cm. in diameter coated 

 with sealing-wax. Both the cylinder and the box of the electro- 

 meter are earthed, while all the other parts including the connecting 

 wire are enclosed in brass tubes which are also earthed, so that 

 they are protected from any electrical disturbances. Moreover, the 

 connecting wire is enclosed in a small glass tube to protect it 

 against those rays which may enter the protecting tubes. The 

 charging and discharging of the brass rod are effected by pushing 

 a small disc D, which can be connected by means of a commutator 

 M with the negative pole of a battery or with the earth at will, 

 against the top of the rod, whose potential is indicated by the 

 divergence of the gold-leaf of the electrometer, which is read by 

 a microscope with a micrometer eye-piece. The leakage of the 

 apparatus itself (that is, the ionization vessel, the electrometer 

 and the connecting wire) when no ionizing agent is at work, is 

 very small ; it required about 40 minutes to decrease the divergence 

 of the electrometer by one division, so that it can be entirely 

 neglected in the following experiments, which treat of the potential 

 fall corresponding to 20 divisions in 1 or 2 minutes. 



§ 2. The amount of ionization due to rays is measured by 

 the leakage thereby produced from the brass rod to the wall of the 

 cylinder. If the current between the rod and the cylinder wall 

 due to the ionization is always in the state of saturation, then its 

 strength is independent of the potential difference between them, 

 and the electricity which escapes, and therefore the fall of potential 

 of the apparatus, is jointly proportional to the intensity of ionization 

 and the time required. Consequently if we apply two ionizing 

 agents separately and produce the same fall of potential of the 

 rod every time, then the strength of ionization of the two agents 

 is inversely proportional to the time required for it. 



In order to see how strong the ionization can be while still 

 maintaining the state of saturation, I calibrated the electrometer 

 (with the ionization vessel and the connecting wire) with a 

 Weston's voltmeter. 



The following is the result. 



VOL. XIII. PT. VI. 



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