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Fig.8 Example curve recorded with the control unit for the Franck-Hertz experiment7.3.3 Calibration of measurement curve• Set the heater voltage to 3.5 V and collectorvoltage to -1.5 V; set the lower limit for theaccelerating voltage to 0 V and the upperlimit to 60 V. Turn up the gain.The oscilloscope screen shows a curve which hasa weakly defined structure in three places. The firstof these structures are of interest here. To emphasisethese places, carry out the following procedure.• Reduce the upper limit of the acceleratingvoltage down to about 35 V.This zooms in on the measurement curve andthe structures become clearer.• In order to display the curve even bigger,raise either the gain or the heater voltage.You may need to change the oscilloscopesettings.• Raise the upper limit of the acceleratingvoltage (to about 15 V) until the curve startsat the edge of the first peak. You may needto increase the gain to display the structuresbetter.• Reduce the upper limit of the acceleratingvoltage (to about 20 V) until the curve endsat the point where ionisation begins.The range of the curve in which the critical potentialslie is now displayed within clearly definedlimits on the oscilloscope screen and thecritical potentials themselves can specifically beidentified.A TELTRON Product from UK3B Scientific Ltd. ▪ Suite 1 Formal House, Oldmixon Crescent ▪ Weston-super-MareSomerset BS24 9AY ▪ Tel 0044 (0)1934 425333 ▪ Fax 0044 (0)1934 425334 ▪ e-mail uk3bs@3bscientific.comTechnical amendments are possible© Copyright 2013 3B Scientific GmbH