IONISATION STANDARDS. 
Epwin 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 ElectricityThrough 
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 2.37x10? a particles per second per one 
gram of uranium oxide. (See Geiger and Rutherford, Phil. Mag. May. XX 
page 391, 1910.) 
The following is a very convenient modification of MeCoy’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 
inate ale 
arranged spirally about it from bottom to top as shown in Fig. 1. These 
shelves are about four centimeters 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 
