Cavallo described his machine in his 1795 Treatise on Electricity.[1] He had examined Bennet's charge doubler of 1787 and found it wanting in several regards, notably in its inconsistent operation and tendency to retain the charge from an earlier experiment.[2] Cavallo resolved to build a better device. His machine consisted of four metal plates supported on a wooden board by posts, of which three were insulating and one conducting.[2]
The charge to be multiplied was applied to the first of these (plate A), which stood on an insulating post. A moveable insulated metal plate (B) was brought close to A (though not permitted to touch it), and then grounded. The charge on A caused charge separation on B due to electrostatic induction. Plate B was then moved away, breaking its earth connection. Since B was insulated, it acquired and retained a small charge opposite in sign to the charge on A.[1] Plate B was transferred by means of an insulating rod to be brought into electrical contact with the third metal plate C which was insulated. Since both B and C were conducting, B would transfer a portion of its charge to C. To maximise the transferred charge, C was placed in close proximity to a final metal plate D, which was earthed.[1][3]
The experimenter would move Plate B repeatedly back and forth, placing it near to A and earthed at one end of its motion, and then into contact with C at the other. With each cycle, charge was drawn from the Earth and added to C. After a suitable number of cycles, the grounded plate D would be removed, and the electrostatic potential on C would rise to approximately the potential of A multiplied by the number of operations.[3]
Cavallo termed his device a multiplier, though 'addition' was perhaps a more accurate description of its operation, as the charge on C was accumulated by successive additions.[1]
Wilson's machine, described by its inventor in Nicholson's Journal in August 1804,[4] was a development on this concept which simultaneously operated two Cavallo's multipliers by means of a pair of reciprocating levers.[5] One side would accumulate the charge of the other, and since the two accumulating plates were connected together by means of a wire, Wilson's machine was a true multiplier, rather than an addition machine.[1] The charge would thus accumulate more rapidly than Cavallo's multiplier and the machine could generate high voltages in a short period of time. It moreover was self-exciting,[5] needing no initial charge to operate, as the small initial charge acquired from contact electrification was enough to start the accumulation process.[4]
