![]() ![]() Very carefully, rotate the glass bulb and observe the helical path of the electrons. Turn on the current through the HC, and observe how the electron beam is bent and forms a circle. Apply 150 V to the anode, which should produce a visible beam. Turn on the heater supply, and allow two minutes for the filament to heat up. Place the black cloth over the HC for easy observation of the electron beam. Check that the polarities agree with the ones marked on the front panel of the apparatus. ![]() With this orientation, the effect of the Earth's field on the HC is zero.Ĭonnect the power supplies and multimeters to the \(e/m\) apparatus, as detailed in the previous section. Rotate the coils such that the plane of the HC lies in the plane of the Earth's magnetic field. To do this, use a magnetic compass to determine the direction of the Earth's field, mark its direction on your work station with paper tape, and record its coordinates. Orient the Helmholtz coils (HC) so as to eliminate the influence of the Earth's magnetic field on the experiment. Connect the Simpson multimeter to the jacks labeled for the voltmeter (item E on the front panel of the EBB) set to the highest voltage scale, and put it in the DC-V mode. Set the Triplett multimeter to the highest current scale, and put it in the DC-A mode. (The HC power supply, the ammeter, and the HC are therefore connected in series.) Plug in the power supplies and the multimeters. To measure the HC current, make sure it passes through the Triplett multimeter in the current mode. Use red cables for positive voltage and black cables for ground. (We use AC.)Ĭonnect the EBB to the power supplies and multimeters (see the figure below). Two jacks (H) for connecting the electron-gun heater to the power supply which can be 6.3-V DC or AC. Two jacks (G) for the high-voltage (~300 V) input to the electrodes of the electron gun. Two jacks (E) for the voltmeter, which measures the accelerating voltage. This should be set in the “e/m Measure” position. The grid is internally connected and does not need a separate power supply.Ī switch (D). The focus knob (C), which allows sharpening of the electron beam by variation of the voltage on the grid of the EBB. (The current can also be changed directly by a knob on the power supply.) The current is measured by a multimeter connected in series with the power supply.Ī current adjustment knob (B) for the HC. ![]() Two jacks (A) for the connection to the Helmholtz coils (HC) power supply. Starting from the left side of the panel, we have: On the front panel of the \(e/m\) apparatus are jacks for the power and the voltmeter, two control knobs, and a switch. The \(e/m\) apparatus may be covered with a black cloth to reduce background light. A large current of 1 – 2 A is needed, but a low voltage of 5 – 15 V is adequate. We use a GW Laboratory DC Power Supply Model GPS-1850 for the HC, which also supplies the voltage for the small light bulb of the illuminated scale. The electric current and voltages are measured with two digital multimeters: the Triplett Multimeter Model 4000 and Simpson Digital Multimeter Model 464. This supply also provides the cathode heater power using a 6.3-V alternating current. A voltage of 150 – 300 V at a modest current of approximately 50 mA is required. The accelerating voltage for the EBB is produced by a Heathkit Regulated Power Supply Model PS-4. The apparatus is fed by three external power sources. There is also an illuminated ruler behind the EBB to facilitate the measurement of the electron-beam radius. The centerpiece of this experiment is the Kent \(e/m\) Experimental Apparatus Model TG-13, which consists of the electron-beam bulb (EBB) and a pair of Helmholtz coils (HC). If there is no electric field, then this relation can be written as The electron's equation of motion is given by the Lorentz relation. The trajectory of the speeding electrons moving through the magnetic field is made visible by a small amount of mercury vapor.Īn electron moving in a uniform magnetic field travels in a helical path around the field lines. ![]() A known current flows through a pair of Helmholtz coils and produces a magnetic field. The central piece of this apparatus is an evacuated electron-beam bulb with a special anode. An electron beam of a specified energy, and therefore a specified speed, may be produced conveniently in an \(e/m\) apparatus. Fortunately, the ratio of these two fundamental constants can be determined easily and precisely from the radius of curvature of an electron beam traveling in a known magnetic field. Measuring separately the electric charge (\(e\)) and the rest mass (\(m\)) of an electron is a difficult task because both quantities are extremely small (\(e\) = 1.60217733×10 -19 coulombs, \(m\) = 9.1093897×10 -31 kilograms). Heathkit Regulated Power Supply Model PS-4.GW Laboratory DC Power Supply Model GPS-1850.Kent \(e/m\) Experimental Apparatus Model TG-13. ![]()
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