576 Dr. A. B. Wood on Long-range 



number of long-range particles from thorium, and to differen- 

 tiate, if possible, between long-range a-particles and high- 

 velocity oxygen atoms. 



§ 2. Experimental Arrangements. 



The experiments carried out in 1916 gave an approximate 

 value of 1 in 10,000 for the number of long-range particles 

 relative to the total number of a-particles emitted by the 

 source of thorium active deposit. Unless a very intense 

 source be employed, therefore, it is not possible to obiain a 

 sufficient number of long-range particles for purposes of in- 

 vestigation. To obtain this intense source of active deposit. 

 the flat tip of a copper wire, 0*8 mm. diameter, was exposed 

 as negative electrode to a strong source of radio-thorium *, 

 until the active deposit collected was in equilibrium with the 

 emanation. The electrodes in the exposure chamber were 

 arranged so as to concentrate the field on the tip of the 

 copper wire, thus ensuring a maximum amount of active 

 deposit collected. When sufficiently active, the wire was 

 removed and mounted, as shown in fig. 1, with the active 

 surface 4'5 mm. distant from a zinc-sulphide screen. Usually 

 the solid angle of the a-ray stream from the end of the wire 

 was limited by a small hole, 1*1 mm. diameter, through a 

 brass plate 1*8 mm. thick. The absorbing screens employed 

 to stop the 8*6 cm. a-particles were placed over this hole,. 

 and could easily be removed or interchanged without dis- 

 turbing the relative positions of the active wire and zinc- 

 sulphide screen. The latter was carried on an extension 

 tube, fitted to the microscope so that it always remained in 

 focus. This zinc-sulphide screen, prepared by Mr. F. H. 

 Glew, was of specially fine grain, the urea under observation 

 by the microscope being completely covered with small 

 crystals. The microscope, of magnifying power 50, covered 

 a field 2 mm. diameter. Under these conditions the maximum 

 number of long-range particles observed was about 10 per 

 minute. In cases where such a small number of scintillations 

 is to be counted it is, of course, necessary to take all possible 

 precautions against slight contamination of screens, etc., by 

 radioactive matter. Consequently, it is essential to make 

 " dummy " experiments, in which the active source is re- 

 moved, as a matter of routine. 



To ensure that the relative positions of the microscope,. 

 ZnS screen, absorbing screen, and active source always 

 remained the same through an experiment, they were all 



* Very kindly lent for these experiments by Prof. L. K. Wilberforce,. 

 of Liverpool University. 



