Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 HARD VALVES Hard valves are those in which the electrodes are sealed in a glass envelope containing a vacuum. The object of this is to exclude as far as possible the complicating effects of ions in the valve. Hard valves are described according to the number of electrodes they contain: diode, triode, tetrode (or screened grid), pentode, hexode, heptode (or pentagrid), octode and nonode. Of these the screened grid, hexode, heptode, octode and nonode valves are of little
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 HARD VALVES Hard valves are those in which the electrodes are sealed in a glass envelope containing a vacuum. The object of this is to exclude as far as possible the complicating effects of ions in the valve. Hard valves are described according to the number of electrodes they contain: diode, triode, tetrode (or screened grid), pentode, hexode, heptode (or pentagrid), octode and nonode. Of these the screened grid, hexode, heptode, octode and nonode valves are of little interest in general electronic work, being used for specialized radio appHcations; the important hard valves for electrobiology are the diode, triode, beam tetrode and pentode. Hard valves employ thermionic emission from a heated cathode. The cathode is heated by current supplied from a special auxiUary circuit which for clarity is usually left out of circuit diagrams. The heating may be achieved in either of two ways. DIRECTLY HEATED CATHODE The cathode is in the form of a thin wire or ribbon of tungsten which is heated by the passage of a suitable current through it (Figure ). It is usually coated with a material which emits electrons richly at relatively low Tensioning spring Cathode Support wire Symbol Lead-in wires Figure temperature—dull red heat—thus reducing the amount of heater power which has to be supplied. This material is usually a mixture of barium oxide and strontium oxide. The thermal capacity of the cathode is low so that to achieve a steady emission the heater current has usually to be direct; a few valves having particularly robust cathodes—capable of holding more heat— are designed for direct heating from They are mostly power rectifier diodes, for in power rectification there is ripple in the output anyway, and a little more due to 100 cycle fluctuation in cathode temperature does no harm. 132
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