Lambrecht Electric Contact Polymeter Joins the Collection
A rare mid-century Lambrecht polymeter that could not only measure humidity but also control it has been added to the collection.
The collection has been joined by an Electric Contact Polymeter made by Wilh. Lambrecht of Göttingen around 1960. At first glance it looks like a classic Lambrecht polymeter: a mercury thermometer above the round dial of a hair hygrometer. But next to the familiar trident-tipped pointer, the dial carries two extra setpoint pointers with electrical contacts, adjusted by a coaxial double knob that passes straight through the glass.
In practice, the instrument worked as a two-position hygrostat. The setpoint pointers marked the lower and upper humidity limits. When the main pointer reached either of them, it closed a low-voltage circuit (max. 24 V AC, 3 VA) to operate a relay that switched humidification on or off, or to ring a bell or light a lamp. Such instruments served in greenhouses, textile and tobacco stores, printing works, museum stores and laboratories, wherever humidity had to be kept within limits, not just observed.
The instrument keeps all the functions of an ordinary polymeter, including the thermometer's saturation vapour pressure scale in millimetres of mercury and the auxiliary scale for finding the dew point. Its crazed dial and oxidised pointers suggest it spent its working life on duty rather than in a display case. A detailed description of the instrument, its construction and the principles of the polymeter is available on its page.