295 



Ionisation Standards. 



Ed TV IX Morrison. 



It is very important under certain conditions in radioactive measure- 

 ments to' have an ionisation standard. (See Rutherford's Radioactive Sub- 

 stances and their Transformation, page 111. article 49.) It is also interesting 

 and profitable for students to study the ionising effects of different thicknesses 

 of radioactive substances. (See McClung's Conduction of EleetrieityThrough 

 Gases and Radioactive, page 131. article 86. Makower and Giger's Prac- 

 tical Measurements in Radioactivity, page 42, article 30. and Millikan and 

 Milles' Electricity. Sound and Light, page 350. experiment 28.) 



McCoy describes a method of making an ionisation standard in the Phil. 

 Mag. May. XI page 176, 1906, and such a standard as determined by Geiger 

 and Rutherford was found to emit 2.37xl0 4 a. particles per second per one 

 gram of uranium oxide. (See Geiger and Rutherford, Phil. Mag. May. XX 

 page 391, 1910.) 



The folloTving is a very convenient modification of McCoy's process ol 

 making such an ionisation standard and a method of preparation of material 

 for student work. A brass rod 36 centimeters in length has a series of shelves 



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o 



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arranged spirally about it from bottom to top as shown in Fig. 1. These 

 shelves are about four centimeteis apart, and are designed to support small 

 brass disks. The brass disks should each be accurately weighed and arranged 

 in order upon the spiral shelves. Uranium oxide is carefully powdered in a 



