The emission of ions from incandescent solids has been studied by many investigators. The value of
e/m
, the ratio of the charge to the mass, for the carriers of negative electricity was first measured by Sir J. J. Thomson in the case of a carbon filament heated in a high vacuum. Other observers have since determined the value of this quantity for the negative ions emitted by different incandescent solids, and all agree that the carriers are negatively electrified corpuscles identical in mass from whatever substance they are produced. The investigation of the nature of the carriers of positive electricity emitted by glowing solids presents greater difficulties, owing to the variability of the amount of the positive ionisation produced under different conditions, the causes of which are not yet fully understood. The first measurements of the value of the specific charge of these ions were made by Sir J. J. Thomson, who found
e/m
= 10
4
/25 for the positive ions from a heated iron wire. Later, experimenting with a strip of platinum foil which had already been heated for some hours in a high vacuum, he obtained the value
e/m
= 10
4
/27 for the majority of the carriers of positive electricity from that metal. The positive ions thus seem to have the same mass, about 26 times that of the hydrogen atom, in the two cases. This value suggested to Sir J. J. Thomson that they might be molecules of CO or of N
2
, either of which has a mass about 28 times that of the hydrogen atom, so that it would be impossible to distinguish between these two gases by a determination of
e/m
alone. When a luminous discharge was passed through the residual gas in the tube after the experiments with platinum, the band spectrum of carbon monoxide was obtained, and it was concluded that molecules of this gas acted as the carriers of positive electricity from the glowing metal.
Laser power meters as an X-ray power diagnostic for LCLS-II