DISCOVERIES BY MEANS OF THE GALVANOMETER, ETC. 485 



polarised by a voltaic current, and was thus enabled to 

 accumulate the power of a small battery. It was by 

 means of a voltaic battery current that Nobili, of Reggio, 

 in 1826, discovered electro-chromy. Becquerel, in 1829, 

 invented the first double fluid battery, having a porous 

 diaphragm. Daniell, in 1836, invented his constant bat- 

 tery, by the use of which, in the same year, W. de la Rue 

 discovered that copper electro-deposited from a solution 

 of cupric sulphate produced an exact copy in reverse of 

 the surface upon which it was formed. Grove's battery 

 was invented in 1839; Smee's in 1840; and the latter 

 was immediately used by its inventor to discover a large 

 number of new facts in electrolysis, and to deposit many 

 of the metals. Golding Bird, also, had already, in 1837, 

 used the battery current to discover that even a feeble 

 current was sufficient to deposit potassium and sodium 

 into mercury. In the year 1834, Faraday, by means of 

 his voltameter, was enabled to discover the great principle 

 of definite electro-chemical action. 



By means of a magnetic needle Ampere, previous to 

 September 18, 1820, discovered that the current in a 

 voltaic pile influences a magnet in the same way as that 

 in the connecting wire ; and, by means of the needle and 

 current, was enabled to invent a galvanometer. Schweigger, 

 of Halle, invented his improved galvanometer during the 

 same year. By inventing also a suitably formed helix of 

 insulated copper wire, Ampere, previous to November 6, 

 1820, was enabled to imitate perfectly the action of a 

 magnet by means of an electric current. Snow Harris, in 

 1831, first used the bifilar suspension for needle magnet- 

 ometers. Pouillet first described his invention of an 

 astatic needle in the year 1832, and thus rendered the 

 galvanometer capable of detecting more feeble currents. 

 It was by means of a combination of a thermo-pile, gal- 



