THE EXPERIMENT

CAUTION

The outside of the cabinet containing the Franck-Hertz tube gets extremely hot when the furnace power is ON. Make all your connections while the furnace power is OFF. You will be severely burned if you touch any part of the cabinet or connector plugs while the power is ON. Remember to turn OFF the furnace power when you complete your experiment. The temperature is controlled from a knob on the side of the cabinet.

Equipment

  1. The Franck-Hertz tube mounted in cabinet containing a heater.
  2. A control unit for the Franck-Hertz tube,
  3. A thermometer
  4. An oscilloscope An X-Y recorder.
  5. Connecting leads and cables.

The tube-heater unit has a diagram of the tube on the front showing the connector locations for the cathode heater, cathode, anode, and collector. The control unit has several channels:

Signal channel

The top knob controls the gain of the collector current circuit. The output display voltage is proportional to the measured current. 1 V output voltage corresponds to an input current of 0.7 a in the minimum sensitivity setting (control knob counter-clockwise), and to an input of 7nA in the maximum (clockwise) setting. Output voltages are of order 10 volts. The bottom knob adjusts the reverse bias of the collector with respect to the anode.

Acceleration channel

The acceleration voltage, Va, is varied from 0 to 70 V with the knob. Va can be a repetitive 60 Hz sawtooth wave for the x-deflection on the oscilloscope (switch in ramp position), or it can be adjusted manually for providing the x-axis signal on the recorder (switch in manual position).

Heater channel

The heater voltage is adjusted with the knob. Increasing the voltage produces more electrons.

Setting up and measuring with the oscilloscope

Make sure all power is switched off and the furnace control knob is at the white line before connecting the tube and furnace module, control unit and oscilloscope as shown below. Va/10 is the x-axis voltage and the collector current output is on the y-axis. Insert the thermometer to the center of the furnace from the top of the tube/furnace module.

Set the temperature control on the side of the furnace to give about 1700 C. This control is a bimetal contact switch that will maintain the furnace temperature with about 6o C of a mean temperature. It takes 10-15 minutes for the furnace to reach this "equilibrium" from a cold start.

Switch on the control unit. Set the Va switch to "ramp". Set the filament heater and reverse bias at about mid-position.

Slowly increase Va from a zero position. Your goal is to produce a Franck-Hertz signal on the oscilloscope screen similar to Fig. 3. This will require adjusting the gain control and the cathode heater control. You will note that the first minimum appears at a voltage larger than the voltage between the succeeding equally spaced minima. This results from the fact that the electrons must surmount the reverse bias voltage as well as a "contact potential" that exists between the cathode and anode. The number of minima you observe is dependent on the furnace temperature. At low temperatures (around 150o C) the voltage difference between maxima and minima are large and the first few minima are clearly distinguished but the current saturates at a relatively low accelerating voltage (Fig. 3a). At higher temperatures, more minima appear, but the initial ones may be difficult to measure accurately (Fig. 3b). You will therefore be able to measure the minima spacings under a variety of conditions. Compute the corresponding average wavelengths and their uncertainties for these different conditions. Which are the best conditions for obtaining the most reliable results? What are some important sources of uncertainty in these results?

Setting up and measuring with the X-Y recorder

The instructor or the TA will go over the operation of the X-Y recorder since most students will not have used one previously. Be careful not to damage the sweep arm mechanism by leaning on it or otherwise mechanically loading it.

  1. Set the furnace temperature to a convenient value.
  2. Connect the signal output to the y-axis and Va/10 to the x-axis. Set Va to zero (full counterclockwise) and the Va switch to manual.
  3. Set the cathode heater midway.
  4. Switch on the power.
  5. Set the signal gain to maximum (fully clockwise).
  6. Adjust the reverse bias so that the signal output is zero.
  7. Slowly increase Va and adjust the gain to keep the signal on the chart.

Again you will have the opportunity to find the best conditions for producing useful data. Discuss these conditions and the resulting data as for the oscilloscope measurement. Include a sample x-y graph in your notebook. Which measurement method do you feel is more reliable? Why? Use error analysis to determine the best value for the energy and wavelength corresponding to the difference between the ground state and first excited state for mercury. Some of the principal lines in the mercury spectrum are 185.0, 253.7, 435.8, 546.1 and 579.1 nm. Does your best value agree with any of them?