Replica of Apparatus of Rutherford and Geiger for Determining the Charge on the Alpha Particle, after Original Apparatus of 1908

Description
In 1908 the New Zealand physicist Ernest Rutherford, working at the Cavendish Laboratory in Cambridge, England, was trying to understand the so-called “alpha particle,” one constituent of the radiation given off by radioactive substances. He had earlier shown that the alpha particle had a positive charge and was heavier than the previously known electron. Rutherford suspected that it was the same as the helium atom.
To confirm this conjecture, he needed several pieces of evidence: the number of alpha particles given off in each radioactive decay, and the mass and electrical charge of the particles. Rutherford and his colleague Hans Geiger developed the apparatus shown here to collect the charge carried by the particles given off by a radioactive source in a given period of time. Separate experiments gave the rate of emission of the particles, from which the charge on a single particle could be computed.
The radioactive material was mounted as a thin film on a holder set in the lower bulb. The emitted alpha particles passed upwards (the apparatus was evacuated to eliminate absorption by air) and struck an insulated collector plate, to which they transferred their charge. The plate was connected to a sensitive external electrometer that measured the charge delivered in a given time. The charge determined was close to twice the electron charge, supporting Rutherford’s hypothesis.
BACKGROUND ON THE RUTHERFORD-GEIGER ALPHA-PARTICLE CHARGE APPARATUS
At the beginning of the 20th century, Ernest Rutherford was studying the distinct types of radiation (alpha, beta, and gamma rays) emitted from radioactive elements. In 1908 Rutherford and Hans Geiger conducted a series of experiments to determine the charge and nature of the alpha particle. Their apparatus is described in the following reference: Ernest Rutherford, Radioactive Substances and their Radiations, Cambridge: 1913, p. 135, section 61, Fig. 28. Their original paper on these particular experiments is: “The Charge and Nature of the α-Particle” by Ernest Rutherford and Hans Geiger, Proceedings of the Royal Society A., Vol. 81, 1908, pp. 162-173. (This paper is reprinted in James Chadwick, ed., The Collected Papers of Lord Rutherford of Nelson, New York, 1963, Vol. 2, pp. 109-120)
Object N-08019 was made at the Cavendish Laboratory, Cambridge, England, and is a copy of the original apparatus preserved in the Museum at the Cavendish Laboratory. See Cavendish Laboratory Museum photo at:
http://www-outreach.phy.cam.ac.uk/camphy/museum/area4/images/cabinet_5.jpg
While both our replica and the original at the Cavendish Laboratory Museum resemble the Rutherford-Geiger apparatus as described in Rutherford and Geiger’s publications, they nevertheless differ in several details, as described below.
As described by Rutherford in the first of the above references, the apparatus had a short cylindrical central glass section with a ground joint at each end, into which fit closed-off glass pieces, one longer than the other. The apparatus was “exhausted to a charcoal vacuum” in order to avoid collisions of alpha particles with atoms in ambient air. Alpha particles emitted from a radioactive source in the lower of these end pieces passed up through aluminum foil mounted in the central section and were absorbed by a collector plate supported on insulators in the upper end piece. The current between the collector and the foil was measured with an electrometer. The rate of collection of alpha particles was calculated from the results of earlier experiments. These particular experiments yielded the charge on the alpha particle. Rutherford and Geiger found that each alpha particle carried a charge that was twice the “unit” charge (i.e., charge of hydrogen “atom”). They were thus able to infer that the alpha particle is an “atom” of helium.
Our replica, object N-08019, is as described by Rutherford in the above references, except that the alpha emitter consists of a brass ring on an insulated stem, with a wire across the ring, to which is soldered an axial wire bearing a rectangular plate at its upper end. And in our replica, there is no foil diaphragm across the central section, as in Rutherford’s description. The collector in our replica matches that as described by Rutherford. Further details on the differences between the Rutherford-Geiger apparatus and object N-08019 are contained in notes in the Curator’s file for this accession. The reason for these differences is not known. One possibility is that Rutherford and Geiger modified their apparatus after carrying out the experiments described in their paper.
Object Name
apparatus, alpha charge
apparatus, alpha charge (replica)
date made
1960
maker
Rutherford
Physical Description
glass; brass; aluminum; wax (overall material)
Measurements
overall: 13 in x 13 1/2 in x 1 15/16 in; 33.02 cm x 34.29 cm x 4.92125 cm
projection stopcock tube from main axis: 6 9/16 in; 16.66875 cm
projection of emitter stem from axis: 6 15/16 in; 17.62125 cm
apparatus mounted on base (axis vertical): 13 in x 13 1/2 in x 4 1/16 in; 33.02 cm x 34.29 cm x 10.31875 cm
place made
United Kingdom: England, Cambridge
ID Number
EM*N-08019
catalog number
N-08019
accession number
224580
subject
Science & Mathematics
Modern Physics
Science & Scientific Instruments
See more items in
Medicine and Science: Modern Physics
Exhibition
The Early Sixties: American Science
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Cavendish Laboratory
Additional Media

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