VIII. MEASUREMENT. 335 



needle is furnished with two index-arms crossing each other at right angles, 

 the four ends of which are bent downwards at right angles, so as to mark 

 the deflection upon a graduation placed round the outer vertical surface of a 

 cylindrical ring. 



This apparatus, for demonstration, is constructed according to the directions 

 of Prof. Petrouchevsky, Professor at the University of St. Petersburg. 



1440. Galvanometer with variable resistance. 



Prof. Dr. R. A. Mees, Director of the Physical Labora- 

 tory of the University of Groningen. 



This galvanometer can be used for hydro-electric as well as thermo-electric 

 currents. By its aid the dependence of the sensibility of a galvanometer on 

 its resistance can be demonstrated. 



1441. Reflecting Astatic Galvanometer, with coils of low 

 resistance, and with telescope and scale, for measuring thermo- 

 electric currents of very low intensity. Focal distance of telescope 

 and scale about three metres. Made by Ruhmkorff, of Paris. 



George Gore, F.R.S. 



1442. Galvanometer, by Colladon, with wires insulated by 

 a special method. This instrument was used by the inventor for 

 measuring the intensity of currents produced by electric friction 

 machines, by electricity from the clouds in 1826, and by elec- 

 tric fish in 1831. Prof. Daniel Colladon, Geneva. 



The inventor used this same galvanometer in 1831, for studying the distri- 

 bution of the electric poles upon torpedoes, and the strength of currents pro- 

 duced by animal electricity. 



Annales de Chimie et de Physique, 1826, vol. 33. 



Peclet, Traite de Physique, 1832, vol. 2, p. 221 to 225. 



Memoires de 1' Academic Royale des Sciences, Institut de France, vol. 10, 

 p. 74. 



1443. Balance Galvanometer, giving indication of current 

 in grains or other weights in scale pans, which may be adjusted to 

 any standard. Invented by exhibitor in 1848. 



William Sykes Ward. 



1444. Galvanometer, designed by Colladon for Currents 

 produced by Electro-Statical Charges. 



Geneva Association for the Construction of Scientific In- 

 struments. 



Apparatus for demonstrating that the electricity drawn from friction 

 machines, from the clouds, &c., produces currents, of which the direction and 

 the intensity are measured by the deviation of the magnetic needle of a 

 galvanometer. 



1445. Marine Galvanometers used on board H.M.S. " Aga- 

 memnon " and the U.S. Frigate " Niagara," in the Atlantic Cable 

 Expeditions of 1858. . Sir William Thomson. 



A light mirror, weighing 30 milligrammes and 9 millimetres in diameter, 

 with single needle cemented to its back, suspended by stretched platinum 



