332 



Intelligence and Miscellaneous Articles. 



similar character. In this case, however, it was the negative glow 

 which was the source of radiation. The machine used was of the 

 Wimshurst type, having 160 3-ft. plates each carrying 16 sectors. 



The experiments were made to determine the effect of the rays 

 from the negative g]ow on a photographic plate. 



Small metallic objects were " radiographed," being placed in 

 front of, but not touching, a sensitized plate. The whole was then 

 enclosed in a zinc box (with a small hole cut in the side facing the 

 source of radiation), which was carefully earthed in order to 

 prevent any charge on the metallic object affecting the plate. A. 

 piece of 1 mil. aluminium-foil was then placed between the source 

 of radiation and the plate, thus completing the metallic sheath. 

 The arrangement is shown below. 



<--Z/nc Box 



TO WIMSHURST 

 MACHINE 



PHOTOGRAPH/C 

 PLATE 



/MIL. ALUM'IN/UM 

 FO/L 



"When these precautions were not observed, brush discharges 

 took place from the points of the metallic object (generally a 

 small wheel) which strongly affected the plate. 



Distinct shadows of the small wheel were obtained after 5 

 minutes' exposure in a darkened room. It was at first thought that 

 these photographs were produced by rays similar to the .r-rays, 

 which had traversed the aluminium-foil ; but it appeared afterwards 

 that the whole effect was apparently due to minute holes in the 

 aluminium-foil, since, when the apparatus was wrapped in black 

 velvet, no effects were produced on the plate. That such effects were 

 obtained, however, seems to show that the negative glow possesses 

 strong actinic power, as shown so conclusively by Dr. Cook. 



A possible explanation of the diminished effect observed by 

 Dr. Cook on an Electroscope placed at a distance from a point at 

 which a Brush discharge is taking place, when an induction-coil was 

 used in place of the Wimshurst machine, seems to be that the 

 electrification produced would depend on the R.M.S. potential- 

 difference rather than on the maximum value, as indicated by the 

 spark-length. Similarly for the mechanical force produced by the 

 wind from the points. 



Tours very truly, 



Blythswood Laboratory, E. W. Maechajstt. 



Renfrew, N.B. 



Jan. 30, 1899. 



