CHAPTER XIII. 



MISCELLANEOUS EXPERIMENTS. 



93. Effect of ionization on the refraction of a gas. It seemed interesting to 

 test this question carefully, although a negative result was to be expected. 

 Accordingly one component beam was surrounded by a thick iron tube, while 

 the other was allowed to travel freely in air, along a path energized by the 

 X-rays. For this purpose the X-ray bulb was placed near the grating and the 

 radiation directed toward the mirror N, the beam GM being inclosed. A thick 

 sheet of lead, i foot square, was placed behind the bulb to additionally screen 

 off radiation along GM. Under these circumstances the ionization along GN 

 must have been enormous by comparison with GM. Quiet ellipses were pro- 

 duced in the interferometer, and the effect of opening the X-ray current and 

 closing it again, alternately, was observed. Not the slightest deformation of 

 the ellipses or any motion of the fringes could be detected. An ionization effect 

 is therefore wholly absent. It might have been supposed, for instance, that 

 the ions present might load the wave of light and produce an appreciable result 

 in the interferometer (cf. fig. 92). 



Since a shift of o.i of a ring would probably have been detected, AAT = 

 0.000005 cm. would have produced a perceptible effect. Hence, since n i 

 is, roughly, equal to A N/e, the value of the ionization effect could not exceed 



The ionization effect can not, therefore, exceed o.oi per cent of /* i. 



To further test this question, the iron tube, i inch in diameter and 138 cm. 

 long, was provided with a fine axial wire about 0.02 cm. in diameter, passing 

 through central holes in the glass plates at the end. The ends of the wire were 

 drawn tight by hard-rubber rods on the outside, so that the tube became 

 a cylindrical condenser. All holes were sealed hermetically with resinous 

 cement. The interference fringes were clearly producible. 



The poles of an induction coil were now connected with the inner wire and 

 the tube, respectively, to alternately change the condenser and discharge it, 

 with the object of strongly ionizing the air within. On partial exhaustion the 

 whole tube became luminous, on account of the discharge, in the usual way. 



The best results were obtained with a plenum of air when but two storage 

 cells actuated the coil. Under these circumstances no sparks passed from core 

 to shell of the iron condenser tube, while the air within was intensely ionized 

 by the silent discharge. On closing the current, from 0.5 to i per cent of the 

 rings was swept inward at once. On opening it, the rings again emerged. This 

 inward motion, however, was in the same sense as the effect of a decrease of 

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