. The Bell System technical journal . se together with their descendants upon the distantplate. In those of the other grand type, they are detected each byitself en route. Being charged particles of great momentum, theycleave through any gas in nearly linear paths, along which quantitiesof ions stay behind.^ The Wilson chamber may be used to make thesevisible, and has indeed already been so used (by Joliot, and by Corsonand Thornton); but another device gave the first and as yet mostinstructive results. This is the ionization-chamber equipped withlinear amplifier and oscillograph. In the first
. The Bell System technical journal . se together with their descendants upon the distantplate. In those of the other grand type, they are detected each byitself en route. Being charged particles of great momentum, theycleave through any gas in nearly linear paths, along which quantitiesof ions stay behind.^ The Wilson chamber may be used to make thesevisible, and has indeed already been so used (by Joliot, and by Corsonand Thornton); but another device gave the first and as yet mostinstructive results. This is the ionization-chamber equipped withlinear amplifier and oscillograph. In the first, the ions due to the pas-sage of a single particle are drawn to a collector and their chargesunited; in the second, the united charge is multiplied by a large fixedfactor; in the third, the multiplied charge produces a sharp sidewisemotion of the oscillograph-beam and the spot which this last the photographic plate the moving spot produces a line, the lengthof which is measured. Instances of these lines appear in Fig. Fig. 1—Records of fission-fragments obtained with ionization chamber, linear ampli-fier, and oscillograph. The short lines due to alpha-particles are lost in the hazydark band beneath. (Courtesy of J. R. Dunning) Uranium is a spontaneous emitter of alpha-rays (this is how theradioactivity of U^^ and U^* is manifest) and so the apparatus willshow kicks even when neutrons are absent. This is an advantagereally, since when the neutrons are admitted and the kicks due tothe fragments appear they are so much the larger that there is nodanger of confusing them with alpha-particle kicks, while these lastmay be pressed into service for calibrating the device. The calibrationreposes upon a theorem of very great value in physics: viz., the (aver-age) amount of energy expended by a fast charged particle in producingan ion-pair is fixed and constant, whatever the charge and mass and ^ This is correct whether they travel as isolated nuclei, or are attended by
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