4.4.4 Experimental procedures
1)Preparation before the test
(1)Turn the power of the tester on and the mercury lamp and preheat them for 20 minutes.
(2)Cover the light-shielding cover of the mercury lamp and the phototube cassette,align the light output port of the mercury lamp cassette with the light input port of the phototube cassette,adjust the distance between the phototube and the mercury lamp to about 40 cm and keep it unchanged.
(3)Connect the voltage input terminal of the photocell cassette with the voltage output terminal(rear panel)of the tester(red-red,blue-blue)with a dedicated connection wire.
(4)Zero adjustment:set the“current range”selector switch to the selected position,after the instrument is fully warmed up,zero adjustment before the test.When adjusting zero,switch the“zero adjustment/measurement”switch to the“zero adjustment”position,and turn the“current zero adjustment”knob to make the current indication“000”.After adjustment,switch the“Zero/Measure”switch to the“Test”position,and you can start the experiment.
Attention:Before performing each set of experiments,the zero adjustment must be carried out according to the above zero adjustment method,otherwise it will affect the accuracy of the experiment.
2)Discussion on the measurement of Planck constant h
Theoretically,measure the UAK corresponding to the cathode current at zero under the light irradiation of each frequency,and its absolute value is the cut-off voltage of the frequency.However,in fact,due to the anode reverse current,dark current,background current and polarity of the phototube,the influence of the potential difference between the contacts,the measured current is not the cathode current,and the corresponding UAK when the measured current is zero is not the cut-off voltage.
The anode is often contaminated during the production process of the photoelectric tube,and a little cathode material is stained.After the incident light irradiates the anode or the incident light reflects from the cathode to the anode,it will cause the anode photoelectron emission.When the UAK is negative,the electrons emitted from the anode migrate to the cathode,which forms anode reverse current.(https://www.daowen.com)
Dark current and background current are the photocurrents generated by thermal excitation and the photocurrent generated by stray light irradiating the phototube.Appropriate measures can be taken to reduce or eliminate their effects during phototube production or measurement procedures.
The contact potential difference between the electrodes has nothing to do with the incident light frequency,which only affects the accuracy of U0 and does not affect the slope of the U0-ν straight line,and has no effect on the determination of h.
In addition,the cut-off voltage is the voltage when the photocurrent is zero.If the sensitivity of the current amplifier is not good enough or the stability is not good,it will bring a large experimental error to the measurement.
The current amplifier of this experimental instrument has high sensitivity and good stability.Because of its special structure,the light cannot be irradiated to the anode directly,and the light reflected by the cathode to the anode is also very low.In addition,the use of new cathode and anode materials and manufacturing processes greatly reduces the anode reverse current and the dark current level is also very low.
When measuring the cut-off voltage U0 of light at different wavelength,the“inflection point method”is too difficult to operate and can be replaced by the“zero current method”or“compensation method”.
The zero current method uses the absolute value of the voltage UAK when the current is measured under the irradiation of each spectrum line as zero as the cut-off voltage U0 directly.The premise of this method is that the anode reverse current,the dark current and the background current are very low,and the cut-off voltage measured by the zero current method is very close to the real value.And the cut-off voltage of each spectral line is different byΔU,which has no big influence on the slope of U0-ν curve,so it will not have a big influence on the measurement of Planck constant h.
The compensation method is to adjust the voltage UAK to make the current zero,then keep the UAK unchanged and block the mercury light source.The current I1 measured at this time is the dark current and the background current when the voltage is close to the cut-off voltage.Let the mercury lamp irradiate the phototube again,adjust the voltage UAK to make the current value as I1,and use the absolute value of the corresponding voltage UAK at this time as the cut-off voltage U0.This method can compensate the influence of dark current and background current on the measurement results.
3)Repeat the same measurement
Continue to repeat the same measurement by selecting another 3 to 4 wavelengths of light at appropriate intervals.
