180 Prof. E. Gerard on the Process of 



ment should be the same at the sea-level all round the earth, 

 varying (at the equinoxes) merely as the square of the cosine 

 of the latitude, except in consequence of local disturbances 

 due to want of uniformity in the condition of the earth's 

 surface, 



Terling Place, Witham, 

 Dec. 1889. 



XVIII. Process of Plotting Curves by the Aid of Photography. 

 By Eric Gerard, Professor of Electricity at Montefiore 

 University, Liege*. 



METHODS of registering automatically are being em- 

 ployed more and more in the " technique " of natural 

 sciences. The indicator serves as a faithful and patient 

 observer when it has to register phenomena of slow move- 

 ment. It is also possible to make use of it to note displace- 

 ments which are too rapid for our senses to observe. The 

 registering of rapid movements is most often effected by a 

 style fastened to a movable arm, which traces on a rotating 

 cylinder covered with lampblack. When the movement of 

 the cylinder is not perfectly regular, and when the law of 

 displacement in terms of its duration is required, the time is 

 registered by the aid of an electric chronograph regulated by 

 a tuning-fork. This process has led to important results ; but 

 it is not easily applied when it is required to mark the path 

 described by very light moving arms, or indicators, with very 

 small forces to move them. In such cases the optical method 

 by mirror-galvanometer, with lamp ; and with the traces fixed 

 by means of photography, is available. This has been done for 

 curves measuring variable electric currents, which have been 

 the subject of research in the laboratory of the Montefiore 

 Electro-technical Institute in the University of Liege. 



For this purpose we require an extremely delicate deadbeat 

 galvanometer, with a movable coil of little inertia (electro- 

 magnetic system of MM. Despretz and D'Arsonval). A beam 

 from an electric lamp was thrown upon a small concave 

 mirror fixed to the movable part of the galvanometer, so as to 

 give an image, focused by a lens upon a registering cylinder, 

 which was covered beforehand with a sheet of paper sensitized 

 with a gelatine emulsion of bromide of silver. The time was 

 recorded simultaneously on the cylinder by means of a second 

 beam of light thrown on a movable concave mirror, the axis 

 of which was placed on the prong of an electric tuning-fork. 



* Communicated by Sir W. Thomson. 





