096 



Aery serviceable supplies the advantage that at the beginning the 

 lijdrostatic pressure of the bath need only be approximately com- 

 pensated, which is done by placing a weight about equal to the 

 pressure on the scale Mj ; the accurate adjustment is made afterwards 

 by regulating the level of the mercury. 



e. The electromagnetic compensation. The tixed coil A\ consists of 

 ■J 275 turns of insulated copper wire, wound on a brass frame, 

 sliding closely over the outside of the enclosure; the coil rests on 

 the ring N^ and is fixed at the top by the screw N^. The movable 

 coil M^ has on the one hand to be as light as possible, on the other 

 it has to produce as great a force as possible; account was therefore 

 taken of the fact that for a given weight it is an advantage to 

 make the radius of the coil large and ihe number of turns small. 

 The coil contains 248 turns ((;/=0.7mm.) in two layers, wound on 

 a thin-walled ebonite tube, which is held between two supporting 

 brass rings J/jiVy in the shape of wheels, which may be clamped 

 to the carrier at the desii-ed height. 



The wire which carries the current to the movable coil passes 

 through and is insulated from the cover G^ and is connected to the 

 clamping screw G^ of the upper spiral spring; the current passes 

 through this spring to the ring by which it is attached and which 

 is insulated with ebonite, along the wire 6^3 to the coil and back 

 through the carrier itself, the rod of the floats, a platinum wire 

 dipping in the mercury, the mercuiy and finally a second platinum 

 wire, which carries the current to the cover. 



The electromagnetic system is calibrated once for all by fixing 

 to the lower end instead of the experimental object a small scale, 

 on which definite weights are placed, and regulating the current 

 until the balance is obtained. 



The level at which the carrier tloats, is read on a glass plate M^ 

 with a scale division in tenths of a millimetre, which is focussed 

 with a microscope L^ magnifying about 40 times. For this purpose 

 a window of thick plane-parallel glass is sealed on to an opening 

 in the glass tube A.. On the side opposite to the microscope behind 

 the tube an electric glowlamp is placed in such a position that the 

 scale divisions are seen light on a half-dark background : in this 

 manner it may be very sharply determined when the cross fibre of 

 the microscope exactly coincides with the division. 



§ 4. The experimental tubes. The substances investigated by us 

 (salts, powdered metal) are all enclosed in glass tubes, concerning 

 which the following may be mentioned. 



