Replica of Rutherford and Geiger apparatus for determining the charge on the alph-particle, after original Cavendish Lab apparatus of 1908

Description:

Rutherford-Geiger alpha-particle charge apparatus 1908, replica of Cavendish Lab apparatus. Object ID EM.N-08019: length 13 in (33.1 cm) x width 13 1/2 in (34.2 cm) x depth 1 15/16 in (4.9 cm)

This object consists of a cylindrical glass body with a tapered ground socket at each end. (Cylinder axis is vertical). On each side, at the middle of the glass cylinder, a glass tube extends out horizontally. The longer of the 2 tubes is provided with a stopcock.

Into the lower ground socket of the main body is inserted a cylindrical glass vessel with a rounded bottom. A glass tube (emitter stem) extending at right angles from the inserted glass cylinder supports an axial brass rod, insulated with red wax. The end of this rod inside the vessel bears a horizontal brass ring, to which is soldered a thick wire across the diameter of the ring. At the center of this is soldered a vertical wire, at the upper end of which is a horizontal rectangular plate.

Into the upper ground socket of the main body is inserted a second cylindrical glass vessel, this one bearing an axial glass tube (collector stem) through which passes an axial brass rod, insulated at each end with red wax. The lower end of this rod bears the collecting electrode, consisting of a short hollow brass cylinder with a disc of foil, possibly aluminum, across its lower end.

History and basic principles

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.

Additional background on the replica of 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.