for Measuring Ionization. 719 



understood by referring to the former description. The 

 apparatus was constructed to fit in between the quadrants 

 and the base of a Dolezalek electrometer. 



A represents a rectangular brass box, closed on all sides, 

 and mounted so as to be free to revolve on the base JB, its 

 horizontal position being adjustable by means of the levelling 

 screws. On top of this box is fastened the electrometer 

 proper E, one pair of quadrants of which is connected to the 

 electrode t, and the other, d, to the electrode v, and to earth. 

 The electrodes t and v serve as supports to the plate e, which 

 forms the upper plate of the standard S. The electrode v is 

 insulated from the plate by a piece of ebonite. Below this 

 plate is fastened the standard, insulated from the brass box 

 by ebonite. The cover g of the standard can be moved out 

 and in by means of the ebonite rod h, which runs through a 

 bushing k. The rod is graduated in millimetres, and the 

 position of the cover of the standard can be read at a glance. 



Contact between the plate e and earth is made by means 

 of the key K, which moves up and down in a brass bushing 

 connected to the case A. This key is operated by the aid of 

 a thread running over a small pulley. 



The testing- vessel X, as here shown, is for use chiefly with 

 penetrating types of radiations, such as /3, <y, and Rontgen 

 rays, though it would serve admirably for measuring the 

 excited activity due to radium, thorium, &c. It consists of 

 a cylinder b connected to, and insulated from, the case by 

 means of an ebonite block. The central electrode a passes 

 through this block and is joined to the plate e. Between 

 the rod a and the cylinder b is fastened a small guard-ring r 

 connected to the case by a small screw running through the 

 ebonite. Electrical contact with the cylinder b is made by 

 means of the binding-post I which passes through the 

 ebonite block. The whole testing-vessel is surrounded by 

 another concentric cylinder n joined to the case. 



If desired, the testing-vessel can be quickly removed and 

 the parallel-plate vessel substituted in its place. The whole 

 apparatus is so constructed that it can be readily taken apart 

 if necessary. 



The potential necessary for the operation of the instrument 

 is furnished by two sets of Volta piles, each consisting of 

 170 elements of zinc and copper disks joined together by 

 moistened blotting-paper. These sets are sealed up in glass 

 tubes and fastened to the sides of the case, one set serving 

 for the standard, and the other for the testing-vessel. The 

 potential of these piles is quite sufficient for saturation with 

 the standard used, being in the neighbourhood of 100 volts 



