718 



Dr. S. J. Allen on a Null Instrument 



balance ionizations in the vessel X over a wide range without 

 the aid of any accessory device, such as condensers in parallel 

 with the quadrants. 



This balance method should prove very convenient for 

 measuring the ionization due to a substance, o£ which the 

 activity was decaying rapidly with time, such as the excited 

 activity of radium. Having once obtained a balance at the 

 start of the experiment, the needle can be kept at rest by 

 continually sliding in the cover of the standard at the same 

 rate as the activity is decaying, and taking simultaneous 

 readings of the time and point of balance. 



Instead of the parallel-plate apparatus X, any other of the 

 well-known types of" testing-vessels for ionization can be 

 substituted with equally good results. 



The complete apparatus as finally designed and constructed 

 is shown sketched in detail in fig. 3. The drawing is not to 



Fig 3. 



exact scale, but the relative proportions as there given are 

 approximate. The principle and arrangement is precisely 

 the same as in the preliminary apparatus, except that all 

 the various parts are made compact and self-contained. In 

 describing the different parts of it I shall make use of the 

 same letters as before, so that their functions will be readily 



