Mr Cooke, Experiments on Penetrating Radiation. 161 



diagram was gradually evolved, additions and alterations being 

 made to overcome working difficulties as they arose. 



The general description of the apparatus is as follows. A 

 central charged rod of piano wire, with a platinum tip, is fixed in 

 a small rod of block amber insulation, which is supported in a 

 brass cup, kept in electrical contact with the case, except when 

 the insulation leak is being measured. The device at the top is 

 to charge and discharge the insulated rod, a plunger with a plati- 

 num disc being employed for this purpose. When not in use 

 a nut may be screwed down to hold it firmly in position, and 

 thus close the air inlet. Outside the plunger is a brass tube, to 

 prevent electrification. This is surrounded by an ebonite tube, 

 outside which is a guard ring, kept earthed during readings. 

 The top of this guard ring is in the form of a disc, to protect the 

 insulation under the flange, on the perfection of which the 

 accuracy of the readings depends. Two brass caps slide on over 

 the top and bottom attachments, and being carefully waxed down, 

 render the apparatus air-tight during runs. The plunger may be 

 connected to the battery by means of a tapered plug, fitting in 



VOL. XIII. PT. III. 



12 



