(1)A beam of positive ions of mercury produced from an arc in mercury vapour and fired upon a surface of nickel produces an emission of electrons.
(2)For a fresh untreated nickel target the electron emission is of the order of 1·5 per cent. for ion energies less than about 600 electron-volts, and rises to about 15–20 per cent. at 2000 electron-volts.
(3)After degassing thoroughly at a red heat the secondary electron emission is found to fall to about half the value observed with a dirty target. Continued bombardment with mercury ions leads to a progressive decrease in the emission, which approaches a final steady state after some hours. In this steady state the ratio is about 2·3 per cent. at 2000 electron-volts. After degassing afresh, the same process is repeated.
(4)Tests were made to assure that the emission is in reality one of electrons from the struck target, and that it is due to the impact of the positive ions.