MODERN SCIENCE. 231 



In other words, a ray of ordinary light passing through 

 Iceland spar is divided into two polarised rays, and by 

 recombining the two polarised rays, common light is re- 

 produced. The NlGOL PRISM is used as a polariser or 

 analyser so that it polarises light, and shows the effects of 

 polarisation ; the polaroscope is another instrument for 

 showing the phenomena of polarised light, such as the colours 

 produced, and so forth. We shall see that Fresnel and 

 Young succeeded in elucidating these beautiful phenomena. 



17/5 1836. Ampere, continuing Oersted's experiments 

 upon the deflection of the magnetic needle, found the north 

 pole of the magnet to act in four different ways accord- 

 ing to the direction of the current, and to simplify the 

 description, he gave the following memoria technica, called 

 AMPERE'S RULE, by which all the various directions of the 

 influence of the current can be remembered : " If an observer 

 will fancy himself to be standing (either on his feet or on his 

 head) in such a manner that the positive current will come 

 out of his mouth and return to his feet, the north pole of the 

 magnet will always be on his left-hand side." Upon this fact 

 depend the signals of the telegraph, as we shall partly see. 

 Ampere invented the galvanometer for determining the exis- 

 tence, intensity, and direction of an electric current. This 

 apparatus, combined with the commutator which will reverse 

 the current, has become one of the chief parts of the electric 

 telegraph : the motions of its needle indicate the signs 

 transmitted. Ampere suggested (1830) the working of signals 

 by the motions of the magnetic needle, and made the first 

 APPARATUS OF TELEGRAPHY, which gradually resulted in the 

 telegraph through the labours of Jacobi, Steinheil, and 

 Wheatstone. Ampere also discovered that two electric 

 currents produce magnetic currents at right angles to them- 

 selves in the air without needing any steel bar to help them. 

 He made another important discovery: as electric currents 

 caused magnetic currents, he thought it possible to turn a 

 steel bar into a magnet by passing an electric current 

 round it and effectively he made ELECTRO-MAGNETS by 

 this means. He was then the founder of ELECTRODYNAMICS, 

 that part of electric science which deals with the attraction 



