418 Prof. J. C. McLennan and Mr. C. L. Treleaven 



on 



upper ends to an insulating amber support and at their 

 lower ends to a non-conducting cross-fibre, also o£ fused 

 quartz and under tension. This cross-fibre was attached 

 to an insulating support of amber, as shown in fig. 1. 



Fig. 1. 



The instrument was provided with a sliding-iube S S, 

 which could be lowered so as to enclose the electrical system, 

 and so reduce to a minimum the volume from which ions 

 could be drawn. With the tube S S raised, the effective 

 volume of the chamber was 2021*1 c.c, and with it lowered 

 31*5 c.c. The fibres were illuminated by light reflected into 

 the instrument by a mirror through a glass window in the 

 back of the instrument. The separation of the fibres when 

 charged w r as measured by means of a microscope provided 

 with a scale. The electrical system was calibrated in the 

 ordinary way with a set o£ storage-cells, and by means of a 

 calibration-curve the changes in the separation of the fibres 

 were transcribed into potential falls. In practice the fibres 

 were charged by means of a Zamboni pile through the 

 intermediary of an insulated sound passing through the 

 walls of the vessel, as shown by the diagram, which could be 

 turned when desired so as to come into contact with a metal 

 piece connecting the fibres to their upper insulating support. 

 All contacts, except in the case of the covering-tube, A, 

 were either soldered or made by fluted joints provided with 



