1] METHODS OF MEASUREMENT 69 
measurements of radio-activity has been given in the last chapter. 
It has there been shown that the essential condition to be fulfilled 
for comparative measurements of the intensity of the radiations 
is that the electrical field should in all cases be strong enough to 
obtain the maximum or saturation current through the gas. 
The electric field required to produce practical saturation 
varies with the intensity of the ionization and consequently with 
the activity of the preparations to be examined. For preparations 
which have an activity not more than 500 times that of uranium, 
under ordinary conditions, a field of 100 volts per em. is sufficient to 
produce a practical saturation current. For very active samples of 
radium, it is often impossible to obtain conveniently a high enough 
electromotive force to give even approximate saturation. Under 
such conditions comparative measurement could be made by 
measuring the current under diminished pressure of the gas, 
when saturation is more readily obtained. 
The method to be employed in the measurement of this ioniza- 
tion current depends largely on the intensity of the current to be 
measured. If some very active radium is spread on the lower of 
two insulated plates as in Fig. 1, and a saturating electric field 
applied, the current may be readily measured by a sensitive gal- 
vanometer of high resistance. For example, a weight of 45 gr. 
of radium chloride of activity 1000 times that of uranium oxide, 
spread over a plate of area 33 sq. cms. gave a maximum current of 
1-1 x 10° amperes when the plates were 4°5 cms. apart. In this 
case the difference of potential to be applied to produce practical 
saturation was about 600 volts. Since most of the ionization is 
due to rays which are absorbed in passing through a few centi- 
metres of air, the current is not much increased by widening the 
distance between the two plates. In cases where the current is 
not quite large enough for direct deflection, the current may be 
determined by connecting the upper insulated plate with a well 
insulated condenser. After charging for a definite time, say 1 or 
more minutes, the condenser is discharged through the galvano- 
meter, and the current can be readily deduced. 
50. In most cases, however, when dealing with less active 
substances like uranium or thorium, or with small amounts of active 
