Section III., 1913. U5] Trans. R.S.C. 



Self Recording Electrometer. 

 By J. Patterson, M.A. (Cantab). 

 Physicist to the Meteorological Service of Canada. 

 P.-esented by R. F. Stupart, F.R.S.C. 

 (Read May 28, 1913) 



Introduction. 



Benndorf* has designed an electrometer which gives a con- 

 tmuous record by mechanical registration and the Cambridge Scien- 

 tific Instrument Co. has a self recording galvanometer on the same 

 principle, called the thread recorder. In the Benndorf instrument 

 the apparatus for producing the registration is operated by an electro- 

 magnet worked by clockwork, while in the thread recorder it is all 

 done by clockwork. It occurred to the author that the recording 

 mechanism of the thread recorder could also be used for a self record- 

 ing electrometer as well as for a galvanometer. In these instru- 

 ments a boom with a counterpoise is attached to the axis of the needle 

 and the excursions of the end of the boom are registered at stated 

 mtervals by a bar dropping and nipping an inked thread or ribbon 

 between the boom and the recording paper, thus leaving a dot, which 

 gives the deflection of the needle at the moment of registration. In 

 this way all mechanical friction is avoided. 



Preliminary Experiments. 



Preliminary experiments were made by roughly constructing 

 models of the electrometer in order to design the size of the quad- 

 rants, length of boom, etc.; that would give the most delicate instru- 

 ment, and at the same time be as compact as possible. Throughout 

 this paper "The sensitiveness of the instrument" refers to the open- 

 ness of scale. An adjustable bifiliar suspension of silver platinum 

 wire was used in all the preliminary experiments. The quadrants 

 were 5" in diameter and ^" between the faces, and a needle of the 

 dolezelek type 4^" diameter was made of silvered mica. The needle 

 was insulated from the boom and was damped by a platinum vane 

 *Akad. Wiss. Wien, Sitzb. d. mathem-natum. cl; c/I. Bd., Abth. Has. 487, 1902. 



