720 Dr. S. J. Allen on a Null Instrument 



each. One of them is also sufficient for charging the needle 

 o£ the electrometer. 



The movement of the needle is observed by means of a 

 spot of light reflected from the concave mirror m onto a 

 scale fastened to the case. 



The whole apparatus is thus self-contained, and, by simply 

 lowering the needle down onto the quadrants to protect it, 

 can be moved from one place to another, and operated with- 

 out any accessories. In making an experiment the case is 

 earthed, thus electrically shielding all the working parts 

 from static influences. 



It was found that the needle was perfectly steady in its 

 movements, and a steady source of ionization could be balanced 

 to one-tenth of a millimetre, which is about one-tenth of one 

 per cent., though of course the accuracy of the instrument 

 necessarily depends on the correctness of the calibration. 



I have not as yet had time to make a very exact cali- 

 bration of the instrument, but the curve B in fig. 2 shows 

 one which is correct to one or two per cent. 



In order to illustrate the working of the instrument, I 

 will give very briefly the results of several well known 

 experiments in radioactivity and Rontgen rays. 



Experiment I. Determination of the strength of a small 

 quantity of impure radium bromide. — This experiment was 

 performed with the preliminary apparatus, the radium being 

 placed on the lower plate b of the testing- vessel X. The 

 middle point of the battery was earthed, and the cover of the 

 standard opened until on separating the quadrants from 

 earth there was no movement of the needle, thus showing a 

 balance of the instrument. This balance-point was found to 

 be at 13*8 millimetres on the scale s, which from the cali- 

 bration curve A corresponds to 16 per cent, of that of the 

 standard. The sensitiveness of the electrometer was about 

 O80, which multiplied by the 16 percent, gives 12*8 divisions 

 per sec. as the " rate of leak " which ought to be observed 

 for the standard, using the instrument in the ordinary " rate 

 of leak *' method. This ought of course to be equal and 

 opposite to the " rate of leak " of the ionization-vessel X 

 observed in the same manner. 



A test of this latter, made by closing the standard and 

 moving the contact m to n, gave a "rate of leak" of 12*5 

 divisions per second, which is in good agreement with that 

 calculated from the balance-point and the calibration curve. 



The sensitiveness as here expressed represents the quantity 

 by which the ordinates of the calibration curve have to 



