PRELIMINARY DATA 



729 



hood, a transient oppositely-directed current is set up in the latter. 

 When the current in the first coil is broken, a current in the same 

 direction is induced in the other coil. 



Fig. 236. 



Fig. 235. A Simple Capillary Electrometer. B bottle containing sulphuric acid; 

 Hg, mercury; E,E', platinum wires. dips into the mercury in the vertical 

 tube, and E' is fused through the bottom of B, so as to make contact with the 

 mercury in B, the other end of it passing out through a small hole in the 

 wooden platform F, on which B rests. F is fastened to the stage of the 

 microscope S by a pin, G, passing through one of the clip-holes, and to 

 the wooden upright D by the pin H. D fits tightly over the microscope 

 stage, but can be moved laterally a little so as to bring the capillary into the 

 middle of the field. /, stem of glass T-tube passing through a hole in D. 

 L, rubber tube connecting the capillary point with the vertical portion of the 

 T-tube. A is a reservoir containing mercury connected by the rubber tube 

 M to I. A can be raised or lowered by sliding it in the clips K. C, magnified 

 portion of the capillary tube showing the meniscus. 



Fig. 236. Capillary Electrometer (after Frey). 



Du Bois-Reymond's Sledge Inductorium (Fig, 237). This consists of 

 two coils, the primary and the secondary, the former having a com- 

 paratively small number of turns of fairly thick copper wire, the latter 

 a large number of turns of thin wire. The object of this is that the 

 resistance of the primary, which is connected with one or more voltaic 



