By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
This experimentâ€™s primary purpose was to show magnetic deflection of electrons within a vacuum diode. The subsequent change in the diode tubeâ€™s anode current equated to some degree of deflection. This particular experiment was a consequence of rebuilding a Helmholtz coil pair used for some unknown application. I was lucky enough to find it years ago at an electronics surplus warehouse in Minnesota. I would drive there during my lunch break and search the isles for â€œreally cool stuffâ€ I could use in various experiments or projects. Secondly, I wanted to verify this apparatus was suitable for a demonstration during one of our science meetings. So, this paper summarizes recent activities in rebuilding the Helmholtz coil and quantifying the magnetic field needed to prevent low energy electrons from reaching the anode in a vacuum diode. Interestingly, an experimenter can use this apparatus or similar versions to determine the electron charge to mass ratio (e/m) using the Hull method.
Apparatus and Modifications
The magnetic flux was produced by a Helmholtz coil assembly connected to a direct current (DC) power supply capable of currents up to 1.25 amperes (A) at ~22 volts (V). The electron source was the tungsten filament in a 371B vacuum diode mounted concentrically within the coils and connected to another DC supply operated at 7A. A 45V battery comprised of five 9V batteries in series supplied the 371B tubeâ€™s anode potential. Instrumentation included two (2) digital multimeters (DMM) â€“ one set to measure Helmholtz coil current (in amperes) and the other for potential (in volts). An analog milli-ammeter was connected in series between the 45V supply and the 371B tubeâ€™s anode. A gaussmeter measured the Helmholtz coil flux with its transverse probe placed perpendicular to the field and immediately above the 371B vacuum diode.
Diode Tube Temperature
Before rebuilding the Helmholtz coil apparatus, I characterized the performance of the electron deflection apparatus comprised of the items listed above along with others listed here. Primarily, I wanted to determine if the tubeâ€™s electron emission was adequate for the experiment given filament currents well below the specified ~10A. In this case, it was important the tube did not become hot enough to approach the maximum service temperature of the polycarbonate mount (115 to 135oC).
Once I set-up the apparatus and checked all connections, I adjusted the filamentâ€™s current to various values and noted the results. The initial test used a Kepco power supply adjusted to provide 7.25A (~3VDC) to the filament. With the tube instrumented with a K-type thermocouple, its initial temperature was 27oC and reached 42oC after 11 minutes. Other tests from 6.25 to 8A showed comparable results. Â In each case, the diodeâ€™s anode potential was 45VDC â€“ there was sufficient electron emission from the filament to maintain anode currents up to 25 milliamperes (mA). Thus, the tubeâ€™s thermal output with a 7.25A filament current would not reach the mountâ€™s maximum service temperature given an ~11 minute demonstration.
Helmholtz Coil Pre-Rebuild Performance
This was the initial experiment to see if the Helmholtz coil magnetic field would deflect the 45V electrons. Though another important aspect of this experiment was to measure the Helmholtz coil resistance to estimate the current required to produce various magnetic flux values. In this case the coilâ€™s total resistance (RT) was 0.34W at 20oC as measured with a DMM. I then connected a DC power supply to the Helmholtz coil pair. The 371B diodeâ€™s filament current was 6.25A and I adjusted its anode current to 10mA at 45VDC. I applied a 0.9VDC potential to the Helmholtz coils - the resulting current was 5.4A. The coils magnetic field (not measured) did not deflect any electrons to a point where the tubeâ€™s anode potential changed. I then removed the tube and mounted a gaussmeter probe in its place (figure 2) to measure the magnetic flux. With a 50A current, the flux was only 8 gauss (@20oC). This test was very brief since the current exceeded the 10A variable transformerâ€™s capacity by a wide margin â€“ I do not recommend this practice. I then repeated the test at more reasonable currents, i.e., 11A and 20A. The corresponding magnetic flux was two (2) and 3.5 gauss, respectively. These results were not adequate for the envisioned experiments. Therefore, I decided to rewind the Helmholtz coils with many turns of smaller gauge wire to increase the number of ampere-turns, thus producing a greater magnetic field.
Helmholtz Coil Rebuild
In rewinding the coils, I estimated each coil could hold 2500 turns of #24 magnet wire (0.020â€ actual diameter) based on a 1â€ x 1â€ coil slot, i.e., 50 turns fit in a 1â€ wide slot multiplied by 50 turns high. Using a wire table and a 50% de-rating factor for bundled conductors, the maximum current capacity was estimated at 3A. Therefore, I rewound the Helmholtz coils on the lathe with AWG #24 magnet wire. It is important to remember the 2500 turns of wire is an ideal result given the coilâ€™s slot width and depth. In practice, this was not the case due to my poor coil winding skills. Additionally, the expedient mechanical counter on the lathe failed to count the revolutions accurately during the ill-fated wire winding process (or fiasco). Someday, I will build a decent coil winding machine. After winding each coil, I applied 30-minute epoxy (15mL) and allowed it to cure while mounted on the lathe and rotating at 150rpm for ~50-minutes. The visual result was adequate â€“ any unevenness in the epoxy coating does not affect performance. I applied the epoxy with a spatula before turning on the lathe to prevent the epoxy from flying off. If the lathe is capable of 120rpm or lower, one can apply the epoxy as the coil rotates.
End of Part 1. Just think, only a week until we post Part 2.
 J.B. Hoag, Electron and Nuclear Physics (3rd Ed.), D. Van Nostrand & Company, Inc., New York, 1948, pp. 30-32.
 Power Supply - Kepco (Model ATE 6-10), DC regulated, 1 to 6-volts (0 to 10A).
 Power Supply - BK Precision (Model 1666), DC regulated, 1 to 40-volts (0 to 5A).
 Gaussmeter â€“ DC Magnetometer - Alpha Labs, 0 to 20kgauss.