-illcurrent pulse is produced. The pulses, recorded by a scaling unit, are a measure of the radioactivity of the sample. Radioactivity is measured in terms of the number of disintegrations per unit of time. The disintegration rate of radium was arbitrarily selected as the standard. Tne unit is called a curie and is defined as the quantity of any radioactive materialhaving associated with 1t 3.7 x 101° disintegrations per second (or 2.2 x 1012 disintegrations per minute). activity of one curie. at different rates; One gram of radium has an Other radioisotopes disintegrate therefore the number of curies per gram varies from isotope to isotope in proportion to the half life and the number of atoms per unit weight. One gram of strontium-90 (Sr9°) which has a half life of about 28 years has an activity of 147 curies. A radioisotope with a shorter half life and/or a lesser atomic weight, would have an even greater specific activity. It is impor- tant to note that the radioactivity per gram is large and that the amount of a radioisotope necessary to be of concern as a potential health hazard is essentially weightless. The amount is usually too small to be determined by gravimetric