ELECTRICAL APPARATUS. 155 



negatively and c positively, while A remains exactly in the same 

 condition as at first, and therefore capable of producing the 

 same effects upon B and c, or upon other conductors which may 

 be substituted for them, any number of times. The ultimate 

 electrification of the conductor B is greatest when the conductor 

 c, with which it is put into communication, while under the influ- 

 ence of A, is the earth. The ELECTROPHORUS, of Volta, affords an 

 important and familiar example of the practical application of 

 the principle here indicated. The same principle is also applied 

 in Bertsch's Electrical Machine, which is essentially an electro- 

 phorus so arranged as to work by a continuous motion of rotation. 



By proper arrangements the conductor B, which, in the manner 

 indicated above can be repeatedly electrified in the opposite way 

 to the body A, can be made each time to impart its electrification 

 to another insulated conductor A', thus electrifying it more and 

 more strongly. By then causing the conductor A' to act in its 

 turn upon B, B will be electrified in the opposite way to A', or 

 similarly to the body A. If B when thus electrified is made to 

 give up its electrification to A, this will become more strongly elec- 

 trified. Thus by letting A and A' act alternately upon B, and 

 each time making this give up to A' or to A respectively the elec- 

 tricity it has received while under the action of the other, each of 

 these bodies can be electrified to a greater and greater degree, and 

 will therefore act with greater and greater intensity upon the con- 

 ductor B, and the other conductor, whatever it may be, with which 

 B is, at each operation, put into communication. This is, in 

 general terms, the principle upon which several instruments of 

 great importance act. 



An instrument on this principle, acting by a reciprocating 

 motion, was described by Bennet in the Philosophical Transactions 

 for 1 787, and in thefollowing year an improved form, acting by a con- 

 tinuous rotation, was described by Nicholson. Further improve- 

 ments in detail were afterwards effected by Bohnenberger. These 

 instruments, however, do not seem to have been used as electrical 



