The Evolution of Electric and Magnetic Physics. 159 



named the " volt," after Volta. The various galvanic cells 

 have singly a pressure of from half a volt to two volts and a 

 half. The lowest pressure that will cause a spark to burst 

 through a thin film of air is about six hundred volts. Light- 

 ning flashes indicate enormous pressures, whose magnitude 

 is only guessed at, perhaps, in millions of volts. The higher 

 the pressure, the greater the distance over which the electric 

 spark will jump. 



The discovery of Volta's pile was no sooner published 

 than Sir Humphry Davy commenced experimenting with 

 it in London, and his example was followed in every direc- 

 tion. In 1807 he announced the decomposition by the gal- 

 vanic current of the alkalies hitherto supposed to be elements, 

 or simple, undecomposable forms of matter. Galvanism 

 now opened a wide field for investigation, and attention for 

 the time almost deserted frictional electricity. The search 

 was not long in danger of subsiding, for the next great epoch 

 was Oersted's discovery of the influence of a current upon a 

 suspended magnetic needle. This was in 1820. He dis- 

 covered that a needle so suspended stands at right angles to 

 an electric current passing near it. Up to this date elec- 

 tricity and magnetism had been studied apart, and had held 

 aloof ; now they met and were linked into one branch of 

 physics. Magnetism alone had not made great advances, 

 but, associated with electricity, it has since developed, prac- 

 tically as well as theoretically, in a manner that has almost 

 outstripped fancy. Oersted's discovery roused the whole 

 scientific world. A few weeks after its publication, Ampere 

 issued the first of those analytical investigations on the sub- 

 ject that have made his name so familiar. While the French 

 school chiefly developed the mathematical consequences of 

 the new facts, the experimental study was actively pushed 

 by Davy and Faraday in England, and Seebeck and Berze- 

 lius in Germany. The first electro- magnets were produced 

 by Arago and Sturgeon about 1825, and these laid the foun- 

 dation of the modern telegraph. The electro-magnet also 

 received special development in the hands of Joseph Henry 

 at Princeton about 1829. This was the period at which the 

 electric telegraph was first inaugurated. Electricity had 

 been suggested for this application as far back as 1753, and 

 Ronalds had made a working model telegraph in 1816, but 

 frictional high - pressure electricity had always been the 

 agent, and the difficulty of insulating it had invariably in- 

 terfered with its practical application. The low-pressure 



