M. Colladon on the Deviation of the Magnetic Needle.. 149 



We cannot conclude this notice, without expressing a wish, 

 that those gentlemen who have not sufficient experience in 

 making meteorological observations, or who may not feel it 

 convenient to devote a whole day to science, will at least en- 

 courage and assist those in their neighbourhood who have the 

 inclination and ability to make them, but who want only the 

 requisite instruments. Those who have not yet sent in their 

 schedules of the 17th July, are requested to transmit them as 

 speedily as possible to Dr Brewster, Secretary to the Royal 

 Society, 10, Coates Crescent, Edinburgh. 



Art. XXI. — On the Deviation of the Magnetic Needle pro- 

 duced by a common Electrical Machine *. By M. D. Col- 

 ladon of Geneva. 



All attempts having hitherto failed to produce a deviation of 

 the magnetic needle by the electrical machine, M. Colladon 

 supposed that this was owing to the employment of too small 

 a charge of electricity, or to imperfectly insulated galvanome- 

 ters. He therefore repeated this experiment with a galvano- 

 meter of 100 turns with two needles, such as that contrived 

 by M. Nobili. The wire of it, says he, is doubly covered with 

 silk, and in order to have a considerable quantity of electricity, I 

 employed a battery of thirty jars, and having a square surface 

 of 4000 square inches. The galvanometer was placed in a 

 separate chamber, and communicated with the battery by two 

 copper wires covered with silk, and suspended by insulating 

 cords. At the end of each wire was soldered a very fine point 

 for drawing off electricity. These two points are the extre- 

 mities of the galvanometer. 



After having charged the battery till its electroscope began 

 to diverge, I put one of the extremities of the galvanometer 

 in contact with the exterior armour of the battery, and hold- 

 ing the other extremity by a glass-handle, I approached the 

 point of it to the bottom of one of the jars. When this was 

 done at four or five centimetres distance, the needle of the 



* This is a brief abstract of the original memoir which was read at the 

 Academy of Sciences on the 21st August, and which is printed in the 

 Ann. de Chim. Tom. xxxiii. p. 62. 



