156 GREAT MEN OF SCIENCE 



medicine glass (no doubt somewhat moist inside) and re- 

 ceived a sharp shock, when he touched the nail with one 

 hand while holding the glass in the other. The effect was 

 further increased when the glass was filled with alcohol or 

 mercury. These observations made in the year 1745 were 

 soon made known in various directions, and produced great 

 astonishment. A year later the same experiment was per- 

 formed at Ley den in Holland, and hence the 'intensifying 

 jar' with which numbers of people now began to experiment, 

 was called the Leyden jar; to be historically correct it should 

 be called the Pomeranian or Kleist jar.^ It was soon given 

 metallic coatings, and great attention was paid to the shocks 

 which it gave, and to its spark effects. Benjamin Franklin 

 showed in 1747, that the two coatings carry opposite elec- 

 tricities. He also remarked, when he made the coatings 

 removable, that the bare glass surfaces are themselves oppo- 

 sitely electrified. This effect of the glass, which goes be- 

 yond the part which it plays as insulator (it is called to-day 

 the residual charge, and is not now of great importance), 

 was at that time regarded as of essential interest, until Wilke 

 and Aepinus, two citizens of Mecklenburg, showed in the 

 year 1762, that the intensifying effect is also present in 

 absence of any glass. They covered two large boards with 

 metal foil, hung them up parallel to one another, and very 

 close together, insulated them, charged one positively while 

 the other was connected to earth, and found that the two 

 proved to be oppositely charged, and when touched simul- 

 taneously with the two hands, gave a shock like that from 

 a Leyden jar. They were thus able to clear up to a certain 

 extent the phenomenon called to-day electrification by in- 

 fluence or induction, and Wilke had already made experi- 

 ments with a rubbed plate of glass and a metal plate, which 

 quite corresponded with the later 'electrophorus' of Volta, 



^ In old writings the name Kleist jar is actually found, and might be 

 reintroduced. 



