Make the connections as shown in the figure:

• Note down the filament voltage from the ‘tube manual and apply it across the terminals of the filament of the tube.
• You should be careful that filament voltage should not exceed the rated value and it should remain constant. L c Connect filament to a low tension battery through key. ‘K,’ rheostat R, and a voltmeter V1 in parallel to note down filament voltage.
• Connect the mercury diode (or tube) to the positive terminal of the potential divider (rheostat) through a micro ammeter (or milli- ammeter).
• Connect a voltmeter in parallel to read the anode voltage
• A Put the plug in key ‘kl’ and pass a current through the filament by applying the specified voltage with the help of rheostat ‘R1’.
• Wait for some time till the filament is sufficiently heated up.
• Close the key ‘k’ by a plug and increase the voltage step by step from minimum to maximum by the sliding contact ‘S’ of the rheostat ‘R2’ and note down the readings of the voltmeter V2 and microammeter (or milliammeter) at each step.
• In the beginning’, the microammeter will not any deflection due to very small value
  of current. When the anode voltage reaches about l0 volts, the some deflection is shown
  in the microammeter.
• Now go on increasing the anode (plate) voltage in small regular steps of 1 volt and each
  time note down the anode voltage ‘Va’ and anode current ‘la’. Continue increasing the anode voltage until the microammeter shows a sudden rise in the anode rapidly.
• Take one or two more readings, but the current should not rise above the full scale current.
• Calculate the logarithms of anode voltage (log Va) anode current (log Ia ) for all the readings.
• Plot a graph between log V(a) and log I(a) such that log V(a) on the X – axis and log I (a) along Y. The curve 1S a straight line in the beginning with a Sudden bend where the anode potential is equal to the ionization potential of mercury.