202 RUHMKORFF INDUCTION COIL (c 1910)
H.W. Cox Ltd/London
Patent No. /16926
61*33*33
Consists of two concentric coils of wire wound on a cylindrical core of soft iron wires impregnated with paraffin wax, all mounted on a hollow base containing a capacitor (which is constructed from sheets of tin foil separated by paraffin paper). Contains a flat spring contact breaker but also has terminals to which a mercury break may be connected. A polarity change switch is missing. The primary (inner) coil consists of two or more layers of thick silk covered copper wire impregnated with paraffin wax placed in an ebonite tube. The secondary coil is divided into sections separated by ebonite disks (to prevent discharge within the coil), each containing many turns of very thin, fine silk covered copper wire impregnated with paraffin wax also encased in ebonite. This is terminated in two discharging rods. When a small voltage (12 volts, say) is placed across the primary coil, a large one is induced across the secondary coil (105 volts). It is possible because of the large number of turns in the secondary coil, the concentrated magnetic field (due to the iron core and the close of the coils), and the abrupt and rapid interruptions (due to the contact breaker). The coil was invented in 1851 by Heinrich Daniel Ruhmkorff (1803-1877), who was a German instrument maker in Paris. It was popular for energizing discharge tubes and in particular for generating x-rays (which were discovered in 1895 by Roentgen). Harry Cox died of x-ray induced cancer.
References:

  1. G. L'E. Turner , Nineteenth Century Scientific Instruments, Uni. California Press, Berkeley (1983), p. 184-5.
  2. A.W. Isenthal and H.S.Ward, Practical Radiography, Second Edition, Dawbarn and Ward, London (1898), pp 36-40.

J.C.

244 FOCUS X-RAY TUBE (c 1900)
6/six/No 37592
33*23*15
Blown glass tube with anode, anticathode and focus cathode. Electrodes are connected to soldered terminals by wires. Side discharge tube with anode and cathode. Sealed, rubber capped exhaust tube. Glass discoloured due to radiation damage or metal deposits. The cathode focuses an electron beam to a small spot on the anticathode, where they are absorbed. This excites atoms in the anticathode, which relax by emitting X-ray photons. X-rays emerge from the side of the tube and since they emanate from a small spot on the anticathode, X-ray images are sharp. The extra anode allows a larger current to pass without damaging the anticathode. The side tube is used to release (previously occluded) gas into the main chamber. This is to counteract the occlusion of gas that occurs during operation and which lowers the pressure until the discharge ceases. Tubes of this kind were used in medicine and for further X-ray experimentation.
References:

  1. R.T. Addyman, Practical X-Ray Work, Scott, Greenwood and Co., London (1901) pp. 94-96
  2. I.W. Isenthal & H. S. Ward, Practical Radiography, Second Edition, Dawbarn and Ward, London (1868)p.p. 57-63

G.P

245 X-RAY TUBE, COLLIMATING TYPE (c 1900)
36*27*10
Has side arm with spark-gap, focus cathode, anticathode but no anode and long side arm with X-ray transmitting glass end. Uses a curved cathode to focus the electron beam onto a Tungsten anticathode. The tube has a high lead content to absorb X-rays except for a small transmission window at the end of a long tube. The combination of small source on the anticathode and small window at the end of the tube produced a collimated beam. Primarily used in crystallography, x-ray spectroscopy, and radiography.
References:

  1. Jaundrell-Thompson, F., Ashworth, W.J. "X-Ray Physics and Equipment", Blackwell Scientific Publications, 1965, ch12.

T.E.

328 COOLIDGE X-RAY TUBE (circa 1930)

VICTOR X-RAY CORPORATION/CHICAGO, ILL, USA

60693

50*18D

Spherical glass bulb (purple from radiation damage) with cylindrical stems carrying the electrodes (cathode end broken).

The Coolidge Tube, first produced in 1913 by W. Coolidge, is the forerunner of all the types of x-ray tubes in common use today. The Coolidge tube was the first type of practical x-ray tube to employ the principle of thermionic emission.

A tungsten filament is used as the tube cathode, and during operation is heated to incandescence by passing a current through it. This causes the filament to emit electrons at a rate dependent on the temperature of the filament. The electrons are then accelerated towards the tube anode by the strong tube voltage. Upon hitting the anode, the electrons are decelerated very rapidly, and shed their excess kinetic energy mostly as heat, and partly as x-ray radiation. To prevent the electron beam from dispersing due to repulsive forces between the electrons, the cathode filament is surrounded by a metal focusing cup at a high negative potential, that has the effect of converging the beam to a relatively small focal area on the anode. X-ray tubes previous to the Coolidge tube (known as Gas Tubes), relied for their electron source, on the tube voltage being strong enough to 'pull' electrons from the cathode. These were accelerated towards the anode, and collided with residual gas molecules purposefully left in the tube, ionizing the molecules and causing the ejection of more electrons. In this way, the required electron beam was built up with a kind of 'avalanche effect'. However, the number of electrons in a beam produced this way, and their energy upon collision with the anode, were both dependent on the gas pressure within the tube, which was rarely stable, and difficult to control. In addition, the number of electrons produced in this way, was, by today's standards extremely small, and hence the intensity of the produced x-rays very low, leading to very long exposure times.

The Coolidge tube, using as it did thermionic emission to obtain a source of electrons, removed the dependency on residual gas for the number and energy of the electrons in the electron beam (an indeed in Coolidge tubes, almost all gas is removed). In fact, as the number of electrons produced depended on the current applied across the cathode filament, and the energy of the electrons on the tube voltage, the Coolidge tube made it possible to easily and independently vary the number of electrons (and hence the intensity of x-rays produced), and their energy (and hence the frequency of produced x-rays). Also, thermionic emission allowed a much higher bound on the numbers of electrons produced, and hence lead to a drastic reduction in exposure times.

References:

1. F. Jaudrel-Thompson & W.J. Asworth, X-Ray Physics and Equipment, Blackwell Scientific Publications, Oxford 1965, pp.401-411.

2. H. Pilon, The Collidge Tube: Its Scientific ASpplications, Medical & Industrial, London, Balliere, Tindall and Cox, 1920.

3. J. Thewlis (Ed), Encyclopaedic Dictionary of Physics, vol. 1, Permagnon Press, 1962.

4. http://www.invent.org/book/book.text/26.html

PF

336 X-RAY MACHINE, FOOT (circa 1945)

Maker unknown

125*78*94

Upright metal box painted white, with three black leather viewing holes on the top all facing inwards. The viewing holes have metal alloy surrounds. Under the middle viewing hole is a brass label with the words: 'PATENT PENDING". There are chrome-plated brass strips around the bottom and running up the front and top, parallel to two decorative handles, which are also made of chrome-plated brass. There is a raised metal platform at the front with a rubber foot plate and part of the main body is cut away to allow for the insertion of feet directly under an x-ray tube, which is hidden in the main body of the machine. Beneath the foot-plate is a fluorescent plate which may be seen from the viewing holes via two mirrors. In front of the viewing hole on the right hand side is a plastic switch connected to a power inlet, with a power cord extension coming out of the bottom right hand side of the box, and a high voltage transformer, leading to the x-ray tube. There is no lead shielding.

This exhibit is on loan from the Powerhouse Museum, Sydney.

Foot x-ray machines were used in shoe stores, during the 1940s and 1950s, to check show sizes, especially for children. The radiation dosages given out by these machines were approximately 10 Roentgens per minute. Their use was discontinued around the latter half of the 1950s.

References: Hilary Irvin, In the Footsteps of Röntgen, Hilary Irvin Productions, Edgecliffe, pg 162-163.

MT

137 DISCHARGE TUBES, SET (c 1910)

54*54*13

A set of sixteen discharge tubes, including several Crookes' tubes and Geissler tubes in a mahogany and pine box with shaved wood padding.

A partially evacuated glass tube containing two metal electrodes, the Crookes' tube exhibits fluorescence when a high voltage is supplied through the electrodes. A few of these in the set were used to demonstrate fluorescence in various materials including calcite, ruby and sea shells. Crookes' tubes were also used to study the straight-line motion and momentum of cathode rays (electrons), best demonstrated by the Crookes' Maltese Cross tube, and their deflections in a magnetic field. Later experiments with Crookes' tubes led to the discovery of X-rays in 1895 and the electron in 1897.

A Geissler tube is a long thin glass tube containing two metal electrodes and a gas, in approximately 0.001 atmospheric pressure. When several hundred volts are applied to the electrodes, the gas ionizes, current flows, and the gas glows with a characteristic colour. Originally used as light sources for studying the spectra of gases, as well as general illumination, the modified Geissler tubes are now mainly used as neon signs.

References: Christie's Catalogue, "The Nicholas Webster Collection of Geissler and Crookes' Tubes and Other Laboratory Apparatus", London 1991.

KCM

Continue the Tour to

Click on this button to return to Physics Home Page