482 SPECIAL METHODS OF DISCOVEKY. 



thirdly, the same number of pieces of card of the same 

 size. The cards were ^steeped in a solution of salt, so as 

 to be moist. He lays upon the table a piece of zinc, 

 places upon it a piece of copper, and then a piece of moist 

 card. Over the card is placed a second piece of zinc, then 

 a piece of copper, then a wet card. In this way all the 

 pieces are piled upon each other in exactly the same 

 order, namely, zinc, copper, card ; zinc, copper, card ; 

 zinc, copper, card. So that the lowest plate is zinc, and 

 the uppermost is copper (for the last wet card may be 

 omitted). In this way there are fifty plates of zinc and 

 copper in contact, each separated by a piece of wet 

 card, which is a conductor of electricity. If you now 

 moisten a finger of each hand with water, and apply one 

 wet finger to the lowest zinc plate, and the other to the 

 highest copper plate, the moment the fingers come in con- 

 tact with the plates an electric shock is felt, the intensity 

 of which increases with the number of pairs of plates in 

 the pile. This is what is called the galvanic, or rather 

 the voltaic pile. It was made known in a paper by 

 Volta, inserted in the 'Philosophical Transactions' for 

 1800. This pile was gradually improved by substituting 

 troughs, first of baked wood, and afterwards of porcelain, 

 divided into as many cells as there were pairs of plates. 

 The size of the plates was increased, they were made 

 square, and, instead of all being in contact, it was found 

 sufficient if they were soldered together by means of 

 metallic slips rising from one side of each square. The 

 two plates thus soldered were slipped over the diaphragm 

 separating the contiguous cells, so that the zinc plate was 

 in one cell and the copper in another. Care was taken 

 that the plates were introduced all looking one way, so 

 that a copper plate had always a zinc plate immediately 

 opposite to it. The cells were filled with conducting 



