The Sumpner reflecting electrodynamometer is the first item in the One Hundred Years of Physics at UQ tour. It is located in Display case S16 in the middle of the back wall of Room 231.
Although this instrument looks typical of electrical instruments from the beginning of the twentieth century, it has special significance. Thomas Parnell, our first lecturer in Physics and later the first Professor of Physics at UQ, used it in work on AC circuit measurements which was reported in the Physics Department's very first research publication in 1917. Such measurements were timely as AC power systems were taking over from DC at that time. You can read the paper from the link below.
The cylindrical brass cover has been removed so you can see the instrument better. It has a circular ebonite base with three levelling feet which carries an electromagnet with a coil suspended between its poles. The suspension also carries a mirror to reflect a beam of light to a scale. This acts as a very long and light needle which is deflected when a current passes through the coil. Since the deflection of the mirror depends on the product of the currents flowing in the coil and in the electromagnet windings the instrument can be used not only for measurements of current but also for electrical power, by making the current through the coil proportional to the voltage across the load.. This applies equally well to AC as to DC circuits as the device accounts for any phase difference between the two inputs.
Parnell’s paper showed how the device could be used as a sort of phase-sensitive detector to separate the resistive (in phase) and reactive (out of phase) components of the impedance of an inductor under test. Although the inventor of the device, Dr Sumpner, was present at the presentation of the paper to the Physical Society of London, Parnell was not, as he was serving as a Gas Officer in France in WW I. The maker of the device, Robert W. Paul, was a leading English instrument maker, perhaps better known as the ‘father of English cinema’. He invented motion picture cameras and projectors and made a number of innovative movies at the same time as the Lumiere brothers. There is a link to one of his films below. But he tired of movies and went back to making instruments, this one among them.
This item is part of the UQ Physics Museum ‘100 Years of Physics at UQ’ Tour
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