Discharge in a fluorescent lamp is observed under a high-frequency electric field applied across the lamp wall. The cl. c. voltage (V_DC) is applied to the electrodes at both ends as usual. The discharge current (I_ie) versus V_DC characters differ according to the position of h. f. electrodes. The principal feature lies in the fact that, when Vnc is kept zero, the h. f. voltage applied at the middle of the tube gives no discharge current, while h. f. electrodes shifted to cathode side give negative discharge current, and vice versa. This phenomenon may be analyzed by assuming two discharge electrodes to form a “double probe”
Under the assumption of Maxwellian velocity distrilnition of ion-and electron-flow, the following experimental formula can be derived:
I_ie=Atan hx₁+Be^e2
By pre-determining the values of I_ie under various gas pressures and at constant d. c. and h. f. voltages. the gas pressure can be measured with ease and accuracy in terms of the position of h. f. electrodes.

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