Mr Wilson, On a Portable Gold-leaf Electrometer, etc. 187 



Fitting tightly over the vertical tube attached to the top 

 of the outer case of the electrometer is a brass tube 1*5 cm. wide 

 in internal diameter and 5^- cms. long. On one side is attached a 

 small tube containing an earthing rod for connecting at will the 

 gold leaf terminal to earth. Opposite it is a wider tube con- 

 taining a sliding condenser to be described later. There is in 

 addition a third side tube (not shown in the diagram) for the 

 attachment of a drying bottle. In experiments on ionisation or 

 radio-activity the testing vessel would be arranged to fix directly 

 on to the top of the wide vertical tube, the testing electrode being 

 screwed into the top of the terminal inside this tube. 



One of the annoyances attending the use of an ordinary gold 

 leaf electroscope for measuring leaks is the difficulty of charging 

 it up to the desired initial potential. One of the uses of the 

 sliding condenser mentioned above is to give complete control 

 over the charging of the gold leaf system. This condenser con- 

 sists of an inner rod fixed at right angles to the terminal of the 

 gold leaf, and surrounding it a concentric tube which can slide 

 parallel to its length to give variable capacity. The rod is about 

 2£ cms. long and its diameter is about 2\ mm. The tube has an 

 internal diameter of about 6 mm. and has a range of movement 

 of 3 cms. The tube is kept at a negative potential by being 

 attached to the inner coat of a quartz Leyden jar like that already 

 described. The displacement of the tube of the cylindrical con- 

 denser can be read on a scale outside the wider tube in which the 

 moving part slides. The condenser is charged by an ebonite rod 

 through a platinum wire, the method being the same as that 

 adopted for charging the inner case of the electrometer itself. 

 The movement of the tube of the sliding condenser is limited by 

 stops, so that when fully in or out its position is definite. For 

 the purpose of charging the gold leaf system the condenser is 

 used as a kind of electrophorus. The slider is pushed in, the 

 gold leaf terminal earthed by the earthing rod, and the slider 

 pulled out against its stops. The leaf is thus left with a positive 

 charge. It is convenient to give the condenser such a charge 

 that the operation of making the earth connection when it is fully 

 .in and then drawing it out to its limit leaves the gold leaf at 

 a convenient part of the field of view of the microscope, its poten- 

 tial being the higher potential corresponding to this position, 

 e.g. 105 volts if the inner case of the electrometer is at 50 volts. 

 When this has once been done the operation of charging the gold 

 leaf to this definite potential of, say, 105 volts may be repeated 

 indefinitely, by simply pushing the condenser in up to its stops, 

 earthing the gold leaf and pulling the condenser out to the ex- 

 tremity of its range again. Thus in measuring the leak from an 

 electrode initially at 105 volts the above operation would be 



