User:Jason O Archer/Notebook/PHYC 307L Junior Lab/2008/10/20

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Charge to Mass Ratio Lab 10/20/08

After powering the equipment, we let the electron gun heat up to charge for two minutes. After this time, we apply a large voltage (measured by the voltmeter to be 196.5 VDC) to the electrodes of the apparatus; this makes a blue glowing beam of electrons shoot straight out from the electron gun. This voltage did not produce a circle large enough to reach around to the filament wires (increasing the coil current only makes a small circle, which changes in color from green to blue to violet to blue to green, with a shift into the UV spectrum for low levels of current), so I increased the voltage to roughly 225 VDC (223.1 VDC by the voltmeter) until a large green circle could be seen reaching around to the filament wires.

For the following combinations of accelerating voltage and coil current, the following radii could be observed; (I visually inspected the radius against a ruler-style guide in the apparatus designed to compensate for parallax. I visually inspected at what I could visually discern as the horizontal diameter of the circle to measure the outer and inner radii on both the left and the right, then averaging those four measurements to obtain the radius.)

Accel. Voltage (V) Coil Current (A) Left Outer (cm) Left Inner (cm) Right Inner (cm) Right Outer (cm) Radius of Deflected e- Beam (cm)
268.4 1.283 4.0 3.5 3.8 4.2 3.9
244.7 1.208 4.0 3.6 3.9 4.1 3.9
290.4 1.180 4.6 3.6 4.5 4.7 4.4
264.2 1.409 3.7 3.5 3.0 3.3 3.4
289.4 2.099 2.0 1.7 1.8 2.0 1.9
283.6 1.330 4.1 3.5 3.9 4.1 3.9
231.7 1.002 3.9 3.6 4.1 4.4 4.0
292.2 1.418 3.6 3.4 3.3 3.5 3.4
255.1 1.380 2.7 2.5 2.6 2.9 2.7
281.2 1.265 4.3 3.8 4.0 4.2 4.1

After collecting this data, we cut the power and connect 2 extra banana cables in parallel from the electron gun electrodes to the deflection plates. We then switch the apparatus switch to electron deflection and restore the power. Using the proper jacks, the beam is deflected upward. Reversing the jacks, the beam is deflected downward. Turning on the current in the Helmholtz coils while the electron beam is deflected downward counteracts this deflection to some extent.