XII. SPECIAL COLLECTIONS. 383 



1741. Model frequently used by Faraday during his re- 

 searches on the rotation of a ray of polarised light by electricity 

 and magnetism. Mrs. Faraday, 



1742. Block of Glass pierced by sparks from an induction 

 coil. Presented to Faraday by M. Ruhmkorff. 1861. 



^ Mrs. Faraday. 



1232a. Daguerreotype Portrait of Faraday. 



Mrs. Charles Cowper. 



1232b. Daguerreotype Portrait of Faraday and Brand. 



Mrs. Charles Cowper, 



17 42 bo Original Apparatus, by Arago and Matteuci. 



Polytechnic School, Paris. 



17 4-2 a. Apparatus constructed by A. De la Rive for demon- 

 strating the rotatory motion which an electric discharge in rarefied 

 gas performs around a magnet. 



De la Rive Collection. TJie property of Messrs. Soret, 

 Pcrrot, fy Sarasin, Geneva. 



This apparatus consists of an electric sliell, perforated by a soft irou 

 cylinder, magnetised by placing it on one of the poles of an electro-magnet. 

 The electric discharge is produced between the extremity of the soft iron 

 cylinder, which is insulated by a glass tube, and a metallic ring which encircles 

 the soft iron. As soon as the soft iron is magnetised, the electric discharge 

 begins to revolve around it. Previous to Ruhmkorff's improvements in 

 induction-coils De la Rive made this experiment with the current of the 

 " Armstrong " machine. 



1742aa. Diagrams and MS. by Henry Cavendish (40 

 sheets), the property of his Grace the Duke of Devonshire. 



The Cavendish Laboratory, Cambridge. 



These form part of the hitherto unpublished papers of Henry Cavendish. 

 The experiments described were made in the years 1771-2-3, and relate to 

 electrical measurements. 



He first establishes the law of the inverse square by proving that if a globe 

 is insulated inside a hollow sphere, and then put in connexion with the hollow 

 sphere, and the whole highly electrified, and if the globe is then insulated and 

 the hollow sphere removed, the globe is found to be without charge. 



He then describes a method of measuring the electric capacities of bodies, 

 and applies it to a globe, to discs of different sizes, to squares of different 

 sizes and substances, and to long cylinders and wires, comparing his results 

 in each case with his calculations. 



1742c. Apparatus devised by De la Hive & Sarasin for 



demonstrating that the electric discharge in a rarefied gas, turning 

 under the power of a magnet, draws with it in its rotatory motion 

 the gas which tranmits it, and all bodies, sufficiently light, that it 

 meets with in its course. 



De la Hive Collection. The property of Messrs. Soret. 

 Perrot, fy Sarasin. Geneva. 



