204 26 



the E. M. F. of one cell. This is the least variation in the potential difference that 

 can be produced by means of the above mentioned battery boxes. 



The increase in the deflection of the leaf effected by such a variation of 

 potential difference amounts to very nearly 3 mm., as seen in the eye piece of the 

 microscope. As it would easily be possible to distinguish ^^ of this, I was enabled, 

 according to equation (4), to detect with this testing apparatus a decrease in the 

 charge corresponding to 4.9x^X3^ = 1.6xl0~^ E. S. units, the capacity of 

 the ionisation chamber being 4.9 cm. in absolute electrostatic units. 



When the ionisation is stronger, a correspondingly greater change is produced 

 in the potential difference (ug — Vb)- By means of the two batlery boxes I was able to 

 make in all ten different changes in the potential difference, viz. 2.9; 2 x 2.9; 3 x 2.9 

 etc. up to 10 X 2.9 volts. In my opinion a greater alteration of the potential than 

 29 volts is not necessary, because, if the ionisation is too intense to be measured 

 with sufficient accuracy by means of a potential variation of 29 volts, we are 

 running the risk of there being an absence of saturation current in the testing 

 apparatus, on account of the potential of the aluminium leaf and connecting con- 

 ductors being restricted. In this case it is more reliable to introduce the active 

 gas in smaller quantities into the ionisation chamber, and in this way avoid testing 

 gases too intensely ionised. 



The chief advantage of the method of measurement just described is that it 

 is only necessary to know the small potential difference between two conductors, 

 i. e. the electroscope vessel and the ionisation chamber, in order to compute the 

 ionisation, it not being necessary to determine the high potential of the inner 

 system of conductors exactly. But so few cells were required in order to gauge 

 the small potential differences, that we could take them with us on the journey. 

 Hence I was enabled to use the microscope instead of the magnifying glass in 

 observing the aluminium leaf, the rate of leakage being measured in this case by 

 comparison with the E. M. F. of the small cells taken with us, instead of by means 

 of a scale, which would have had to be ganged every time the testing apparatus 

 was set up. 



The observation is preferably made by means of the microscope than by 

 means of the magnifying glass, as the measurement can be made with greater speed 

 without diminishing the accuracy. It is of great importance that the measurements 

 be made expeditiously in the study of radioactive substances, as they are generally 

 of an extremely changeable nature. Microscopic observations also save time, when 

 the discharge through slightly ionised gas is to be determined. 



Although the leaf can only be seen within a very restricted potential interval 

 on account of the magnifying power of the microscope, the special method of 

 observation here employed renders it possible to use the microscope for measuring 

 at least as great a potential fall as a magnifying glass, i. e. much greater than can 

 ordinarily be measured by the microscope. 



From this it is evident that this method of observation is equally suitable for 



