687 



The needle-sysfem is arranged so as to let vibrations, which act 

 on the system from without bring about as little disturbance as 

 possible'). In order to obtain as much symmetry of inertia as possible 

 with respect to the suspension wire, a disc of aluminium of the 

 same size was affixed behind the mirror by way of counterweight. 

 Moreover particular care was bestowed on the shape of the thin 

 connecting hooks between rod and wire; finally the planes of mirror 

 and needle were placed perpendicularly with each other ^). With 

 these precautions a fine, restful motion of the needle could be 

 successfully obtained. The brass tube with glass window that encloses 

 it, is fitted at the top with an ebonite torsion head and with an 

 arrangement which makes it possible to raise or lower the upper 

 needle without making it turn round. 



The following still requires to be said about the arrangement 

 how and the way in which the three rods (cf. especially Fig. 4) 

 can touch the needle or giiardring through the ebonite: rod l^ is 

 the one that, touching the lower needle h -\- d, brings it to the 

 desired potential before measuring; at the beginning of the measure- 

 ment this contact is stopped and the charge of the ionivsation current 

 can be carried to the then insulated needle. The rod is made of 

 brass and fitted with a subtle platinum point, in order to make sure 

 of a good contact with the platinum needle. In order to bring about 

 the insulation of the needle from the observer's place behind the 

 telescope the rod has been placed in a brass tube which can be 

 pushed tightly into the ebonite; this tube has been shut off at both 

 ends by small brass covers each with a round opening through 

 vv'hich the rod can pass freely without much friction. Round about, 

 the rod between the outer small brass cover and a thicker part of 

 the rod a steel spiral spring has been placed which is tightened 

 when the rod is drawn out. Now, the arrangement is chosen in 

 such a way as to make a weight which hangs by a cord over a 

 pulley draw the rod out and consequently break the contact with 

 the needle, whereas by raising the weight the spring reestablishes 

 this contact; this raising and lowering of the weight can be brought 

 about from a distance by means of a cord over a pulley. 



Rod /,, whicli brings the guardring to potential, is fastened in 

 an ebonite tube, which has been fixed by means of an enclosing 

 small brass cylinder into the ebonite perforation. Rod 4 does only 

 duty when capacity has to be measured (Harms-method); it is made 

 of brass with a platinum point; it is insulated from the enclosing 



1) Cf. a.o. H. E. J. G. DU Bois and H. Rubens: Wied. Ann. 48 p. 236, 1933. 

 ') In Fig. Ill these planes were put parallel to each other for clearness' sake. 



