. Electronic apparatus for biological research . Original contact /u\ line ^°' Original contact (c) New Hg line line Figure The effect of superimposing a proportioning head on a mercury- contact thermostat. (a) Unmodified thermostat; movement of the mercury meniscus {equivalent to temperature fluctuation) across the stationary contact position; {b) the contact is now oscillated by the proportioning head and its movement superimposed on the previous meniscal fluctuation, the intersection of meniscus and needle still determines the on I off periods, 'on' being indicated by the thickened pa
. Electronic apparatus for biological research . Original contact /u\ line ^°' Original contact (c) New Hg line line Figure The effect of superimposing a proportioning head on a mercury- contact thermostat. (a) Unmodified thermostat; movement of the mercury meniscus {equivalent to temperature fluctuation) across the stationary contact position; {b) the contact is now oscillated by the proportioning head and its movement superimposed on the previous meniscal fluctuation, the intersection of meniscus and needle still determines the on I off periods, 'on' being indicated by the thickened parts of the curve; (c) condition of final control, the more even distribution of heat in time has achieved a much smaller temperature fluctuation are about equal (Figure ). The intersecting 'mercury line' will have to move appreciably across this form of oscillation in order to change the ratio and produce compensation; the sensitivity will thus be low. If the apparatus is, on the other hand, working near to its upper or lower limits so that the oscillation curve is cut near its peaks {Figure ), the ratio will be rapidly changed by a small shift in the temperature line—so much so that over- compensation and instability may result. Uniform and adequate compensa- tion over the entire available temperature range can only be obtained from a triangular type waveform {Figure ) which is not so readily obtainable. The wave front must be appreciably steep, but the more rapid within reason is the frequency of its oscillation the less need is there to approach the triangular form. However, one must still retain a sense of proportion about this, for presumably heater currents are being switched via relays, whose frequency of operation—and life—are limited. Instability in mark-to-space control—Over-compensation may lead to in- stability—the regular form of large temperature cycles which may occur under these circumstances is called 'hunting'. It can often be cure
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