352 Mr. J. H. J. Poole on the 



on occasion sensitivities up to 130 divisions per volt were 

 used. To maintain the charged plate of the electrometer at 

 the requisite voltage, a battery of small Ever-Ready dry cells 

 was employed. This battery was carefully insulated from 

 earth, the individual cells being stood en a layer of paraffin- 

 wax, and its voltage was found to remain constant over long 

 periods so long as no current was taken from it. To protect 

 it in case of an accidental short-circuit due to the gold-leaf 

 touching the charged plate, a water-resistance is inserted 

 between it and the latter. A switch is also inserted for 

 convenience in working with the instrument. 



For earthing the gold-leaf when requisite, a copper rod 

 dipping into a strong Ca(Jl 2 solution contained in the small 

 copper cup supplied with the instrument was used. "When 

 it was desired to isolate the gold-leaf, the copper rod could 

 be lifted by a silk thread passing over a suitable arrangement 

 of pulley-wheels. On the whole this key proved fairly 

 satisfactory, but at times it was inclined to give too big 

 an initial charge to the gold-leaf. Usually, however, after 

 two or three attempts, a small enough displacement could 

 be obtained. 



In order to conveniently measure the sensitivity of the 

 electrometer, the copper rod of the earthing key was not 

 directly connected to earth, but was connected to a two-way 

 switch so that a known voltage could be applied to it by 

 means of a volt-box. Thus the sensitivity of the electro- 

 meter can be quickly determined at any time, which is 

 essential as any small accidental shake of the instrument 

 may alter it considerably, and for this reason it is necessary 

 to test it fairly frequently. 



A separate sketch of the final form of testing-chamber is 

 shown. The chamber was constructed of brass, and made 

 in two parts so that it could be easily taken apart to insert 

 the material under test. The joint was carefully ground 

 with carborundum-powder and tallowed, in order to insure 

 the vessel being air-tight when assembled. It could be 

 exhausted through a side tubulure, which was connected 

 to a Geryk pump and manometer. Pressures down to 

 about 3 mm. of mercury could be maintained in the testing- 

 chamber. 



The black pigment or other material is placed on the 

 small brass testing-plate. This plate is carried on a copper 

 rod which projects slightly from the bottom of the vessel 

 and is connected to the electrometer by a fine wire. The 

 wire is surrounded by an earthed shield to prevent any 

 stray electrostatic effects due to the observer etc. affecting 



