689 



as possible, then the bottom plate must be replaced by a ring (cf. 

 Fig. 5) which bears on the lower side in two places diametrically 

 opposed, two flat brass rails, along which one can slide a deepened 

 bottom plate with the sides dovetailed, which forms the bottom of 

 the ionisation cylinder. A second plate, fashioned in the same way, 

 can be slided along the same ways and replace the first. The ring 

 is pressed against the cylinder by two clamping screws. 



Some particulars on insulation and arrangement. 



In order to make sure that the ambroid really insulates the lower 

 needle, several experiments were still made; thus a tensior» of -j-lO 

 Volt was given to the guardring, ƒ brought to the same tension as 

 b: (0 Volt) in order to avoid an ionisation current, then è insulated, 

 so that the charging of (^ could only be the consequence of a transition 

 of charge from the guardring via the amber to h. With a sensitive 

 state of charge the needle displacement amounted to no more than 

 1 a 2 mm. per minute. If we take into consideration that in measur- 

 ing, the difFerertce of tension between h and the guardring is very 

 small — the latter is kept at T^:=:0 — and that the rise in potential 

 of b amounts during the measurement only to a small fraction of 

 a Volt, this will sufficiently prove how excellently the ambroid in- 

 sulates the needle, and that the leak it causes is of no account. 

 As to the arrangement of apparatus, storage-battery etc., it is such 

 as to make it possible, to perform all the manipulations necessary for 

 the preparation of the measurements from the place at the telescope. 



First of all we find here within the observer's reach the storage- 

 battery from which our wires start, in order to bring a, b, c, and 

 / to potential. The connection with a, b, and ƒ is direct, as these 

 conductors are always charged to a potential given by a whole 

 number of accumulators; c on the other hand receives exactly that 

 potential wanted to bring the needle back again to its untwisted 

 state after having charged a. Therefore the desired potential is 

 obtained by means of an adjustable laboratory rheostate working 

 as a simple tj'pe of potentiometer through which a small current 

 is carried of an accumulator, whose one pole is in connection with 

 a storage- battery. Looking through the telescope at the position of 

 the needle, one can at the same time regulate the tension at will 

 by adjusting the rheostate. 



If, in this way, some state of charge has been given to the appa- 

 ratus, and ƒ brought to potential, then the measurement can be 

 started simply by insulating /; from a distance with the assistance 

 of the pulley-system described above. 



