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D330-26 · Miscellaneous
c.1960
Object Overview
This is a Lambrecht Electric Contact Polymeter, an instrument made by the renowned Göttingen firm of Wilh. Lambrecht. The company's history goes back to the last quarter of the 19th century, and its founder's name stands behind the very concept of the "polymeter". Outwardly the instrument is hardly different from a classic wall-mounted polymeter: a mercury thermometer above the round dial of a hair hygrometer. In essence, however, it is a two-position hygrostat with alarm signalling.
Besides the main indicating pointer, it has two setpoint pointers fitted with electrical contacts. These were used to set the lower and upper limits of permissible humidity. When the humidity went beyond these limits, the instrument closed a circuit and operated a relay coil: it switched humidification on or off, or gave a signal with a bell or a lamp. Such instruments were installed in greenhouses, textile and tobacco warehouses, printing works, museum stores and laboratories — wherever humidity had to be monitored and maintained.
hygroscopicity
aluminium, glass, hair, mercury, steel
In Lambrecht catalogues, instruments of this type appear as "Electric Contact Hygrometers, Low-Voltage Type, No. 880/881". The example described here belongs to a later version of this line: both setpoints are adjusted with a single coaxial double knob brought out through the glass.
The instrument is mounted on a vertical U-section metal bracket finished in black lacquer. On the back there are keyhole slots for hanging it on a wall.
On the front of the bracket, a mercury thermometer in a glass cylinder is held by two clips with eyelets. Below it sits the round case of the hair hygrometer.
The sensing element of the hygrometer is a bundle of human hair. It is stretched vertically inside the bracket's channel, behind the thermometer, between tubular spacers, and is thus protected from accidental contact. The bundle is almost as long as the bracket itself, which gives a noticeable pointer travel for a negligible relative elongation of the hair. The upper end of the bundle is anchored in a fitting with an adjusting screw, used to set the readings during calibration. The lower end passes into the hygrometer case, where it turns the pointer arbor through a lever mechanism. A spiral spring keeps the bundle under constant tension.
This arrangement — thermometer and hair side by side on an open bracket — is no accident. Both sensors are in the same air at the same temperature, which is a precondition for a correct dew point calculation.
The hygrometer case is round, with a wide aluminium bezel of concave, "spool-shaped" profile. This shape was produced by spinning soft metal. The bezel holds a thick flat mineral glass with a bevelled edge. The glass is drilled in its lower part: the arbor of the adjusting knobs passes through it.
No screws or adhesive are used. The glass is held by point indentations made with a centre punch along the edge of the bezel: the metal is pushed inwards at two opposite points and so clamps the glass. In the same way, a row of indentations along the rear edge fixes the bezel to the base of the case. This technique is designed precisely for ductile aluminium: on a hard plating, such dents would cause cracks and flaking. Over time, the polished surface of the bezel has developed fine grey-white pitting, a characteristic trace of aluminium oxidation.
The dial is metal and made by a simple post-war method. Instead of fired enamel, it has a white paint finish, with the scales and inscriptions printed directly onto the painted ground. Age and a humid working environment have left a network of craquelure in the paint and small losses of the coating. For an instrument that served in an industrial room, this is a natural patina.
The dial carries two concentric scales.
Near the arbor there is a small trident-shaped diagram with the figures 20, 15, 10, 5 and 0. It indicates which prong of the main pointer to read on the upper scale at different air temperatures (in effect, depending on the season).
The main pointer is thin and black and ends in a trident. It is linked to the hair and shows the current humidity. At the same time it serves as the moving common contact: a bent contact tab is fixed to it near the arbor.
The two setpoint pointers are metal strips, now with a film of oxidation, fitted at their ends with light-coloured plastic flag caps for easier setting. From each one projects a thin wire contact finger, bent into the plane of the main pointer. The left setpoint pointer defines the lower limit, the right one the upper limit.
The setpoint pointers are adjusted with a coaxial double knob of dark phenolic resin: the outer knurled ring turns one pointer, the inner knurled cap the other. The knob passes through a hole in the glass on an insulating washer, so the setting can be changed without opening the instrument.
Current reaches the moving parts through spiral hairsprings at the arbor and through flexible coiled wires in PVC insulation, one white and one green. The coiled lay allows the wires to follow the rotation of the pointers freely, without hindering their movement or loading the mechanism.
A three-pole black plastic screw terminal block is fitted to the back of the case. Next to it, the markings "−", "0" and "+" are written in white:
Below are the rating data: operating voltage max. 24 V AC, switching capacity max. 3 VA. This is the low-voltage version. The instrument was powered from a step-down transformer, and its contacts were rated only for the small current of a relay coil or a signal lamp.
Hair changes its length with relative humidity: it lengthens in moist air and shortens in dry air. Through a lever, these changes turn the main pointer.
Using the knob, the operator set the setpoint pointers to the limits of the required range. For example, if 55–65 % had to be maintained in a room, the left pointer was set to 55 and the right one to 65. While the humidity stayed within this band, the main pointer moved freely between the setpoints and the circuit remained open. When the humidity fell to 55 %, it touched the contact finger of the left setpoint pointer and closed the "0"–"−" circuit; when it rose to 65 %, the "0"–"+" circuit.
Hair develops only a tiny force, so the contact here is a light touch, with no contact pressure and no snap action. Such a contact cannot switch a humidifier or a fan directly. It merely gave a signal to a relay, hence the modest 3 VA. In a typical circuit, the lower contact energised a relay, which latched and started the humidification. The upper contact released the relay and stopped it. The result was two-position control with a hysteresis equal to the distance between the setpoints. In the simplest case, a bell or a lamp was connected to the contacts, and the instrument simply alerted the staff that the humidity had gone beyond the set limits.
At the same time, the instrument kept all the functions of an ordinary polymeter: it showed humidity and temperature and made it possible to determine the dew point.
The thermometer is a mercury thermometer of the enclosed-scale type. The mercury capillary and the milk-glass scale plate are sealed inside an outer glass tube that protects the scale from dust and moisture. According to Lambrecht catalogues, polymeter thermometers were made of hard Jena glass. The tube ends in a clear glass ball at the top and a mirror-silvered one at the bottom. At the lower end of the scale plate, on its reverse side, are the inscription "Wilh. Lambrecht, Göttingen", the triangular company emblem and the number "G 1258".
The thermometer has two scales:
The polymeter was invented by Wilhelm Lambrecht, the founder of the Göttingen firm. Its name ("multi-meter") reflects its essence: by combining a thermometer and a hair hygrometer in a new way, the instrument made it possible to obtain almost all hygrometric quantities without tables or calculating aids. Lambrecht himself called it a "little meteorological observatory". The firm insisted that only Lambrecht polymeters were originals and that instruments by other makers were imitations. The name was later registered as the trademark Lambrecht-Polymeter®.
A classic polymeter makes it possible to determine:
At a given temperature, air can hold only a certain amount of water vapour in invisible form. Once it reaches this limit, it is saturated. Relative humidity shows what fraction of this maximum amount the vapour actually represents. At 100 % the air is saturated; at 50 % it is half saturated. The instrument shows this value directly, with the pointer on the lower scale.
This is shown by the thermometer. The catalogues stressed that the officially tested polymeter thermometer was as accurate as a clinical thermometer.
Air is a mixture of gases, and water vapour contributes its own share to the total atmospheric pressure, its partial pressure (in older terminology, "vapour tension"). The warmer the air, the more vapour it can take up. The maximum pressure rises much faster than the temperature: about 4.6 mm Hg at 0 °C, about 8 mm at 8 °C, about 17.5 mm at 20 °C and about 23.8 mm at 25 °C. The second scale of the thermometer gives it without any calculation.
The actual vapour pressure equals the maximum pressure multiplied by the relative humidity. Example: at 20 °C and 60 %, this gives 17.5 (from the thermometer scale) × 0.60 ≈ 10.5 mm Hg. Lambrecht particularly stressed the importance of this quantity for barometric observations: with the barometer reading 750 mm, it makes a considerable difference whether 14 mm or 4 mm of it is due to water vapour.
This is the difference between the maximum and the actual vapour pressure. In the same example, 17.5 − 10.5 = 7 mm Hg. The value indicates the drying power of the air: the greater the deficit, the more vigorously the air draws moisture from everything in it — grain, tobacco, wood, fabric and human skin.
Here the polymeter makes use of a convenient physical coincidence: in the ordinary range of room temperatures, each millimetre of vapour pressure corresponds to almost one gram of water per cubic metre of air. So the vapour pressure found in millimetres gives the absolute humidity in g/m³ almost directly.
Example. A room of 5 × 4 × 3 m has a volume of 60 m³. At 20 °C the maximum vapour content, as already mentioned, is about 17.5 g/m³. With the hygrometer reading 60 %, the vapour pressure formula given above shows that each cubic metre of air contains about 10.4 g of water; multiplying this by the known volume of the room gives about 624 g of invisible moisture in it. Saturated air in the same room would hold about 1,040 g. It can therefore still take up about 414 g of water. If wet laundry holding two glasses of water is brought into such a room and the room is not ventilated, the laundry will hardly dry at all.
Another example from an old Lambrecht brochure is a school classroom in winter. Outdoors, at 0 °C and 80 %, a cubic metre of air contains about 4 g of vapour and can take up only about 1 g more. The same outdoor air, warmed in the classroom to about 17 °C, still contains 4 g, but its limit is now about 14 g. The relative humidity drops to almost 29 %, and the air eagerly absorbs water. The brochure concluded that the pupils' hoarseness and coughing were caused not only by school dust but also by over-dried air.
The dew point is the temperature to which air must be cooled to become saturated. Below it, the invisible moisture condenses into dew, fog, clouds or hoarfrost. The polymeter makes it possible to find it in two ways.
The precise method — using the thermometer. The calculated actual vapour pressure is located on the "Sättigungsdruck" scale and the temperature opposite it is read off. In the example, 10.5 mm stands opposite 12 °C, so the dew point is about 12 °C.
The quick method — using the upper dial scale. The hygrometer pointer indicates an "auxiliary number", the difference between air temperature and dew point. This number is subtracted from the temperature. At 12 °C and an auxiliary number of 3, the dew point is 9 °C. More examples from the original instructions: at 10 °C and a number of 6, the dew point is 4 °C; at 3 °C and the same number, −3 °C; at −3 °C, −9 °C.
The trident. At the same relative humidity, the difference between temperature and dew point depends on the temperature. That is why the pointer ends in three prongs. At a mean daily temperature of 20 °C the left prong is read, at 10 °C the middle one, at 0 °C the right one; intermediate temperatures are interpolated between the prongs. The instructions put it in everyday terms: the left prong in summer, the right one in winter, the middle one in spring and autumn. The trident diagram near the arbor of the dial, with the figures 20, 15, 10, 5 and 0, gives the same guidance in graphic form.
A hair hygrometer needs periodic checking, and Lambrecht proposed an elegant method for this that required no reference psychrometer. Hair thoroughly wetted with clean water is about 1/2000 of its length shorter than in saturated air. Therefore, when the hair is wetted, the pointer settles not at 100 but at 95, regardless of how humid or dry the air was during the check.
The procedure is as follows: the instrument is hung where water can do no damage, and the hair and the inner walls are thoroughly wetted with a sprayer — or, in the old instructions, with a pigeon feather. After an hour the operation is repeated. If, after 15–20 minutes in the wet state, the pointer stands at 95, the instrument is correctly adjusted. If not, it is corrected with a small screwdriver using the adjusting screw at the top of the instrument.
In very dry rooms it was recommended to wet the hair about once a month: after a long time in dry air, hair lengthens and does not return to its original state until it has spent some time in a humid environment. For locations with sharp fluctuations or extremely low humidity, the firm offered a special patented PERNIX version.
Lambrecht brochures supplied the polymeter with a set of rules attributed to a Dr Troska and presented as proven by many years of observation:
The rules came with notes:
The old brochures also gave the polymeter a role in hygiene. Humidity of 40 % and below dries and irritates the skin and makes people easily agitated. If the dew point rises above 12 °C, especially quickly and with high relative humidity, the lungs and skin give off moisture less easily, producing the familiar feeling of stuffiness and oppression. The same reasoning was used to explain why people from the mountains and the north feel unwell on humid plains, and why consumptives were sent to cold Davos or to dry Egypt.
For industry, the polymeter was above all a monitoring instrument. The catalogues list food production, slaughterhouses, cold stores, tobacco and cigarette factories, breweries and malt houses, granaries, spinning and weaving mills, dye works, paper mills, timber-drying plants, printing works and telecommunications facilities. Some businesses are harmed by dryness (tobacco), others by damp (grain, meat), and others need a precisely defined humidity, neither more nor less (spinning mills).
The brochures stressed that "dry" and "humid" are relative terms: air that is dry for a tobacco store would be damp for a meat cold store. For this reason, vegetable hygroscopes made of straw, wood or wheat awns, which only show "damper" or "drier", were considered unsuitable. What was needed was an instrument that gave a number. The importance of that number is well illustrated by a curious episode from trade history: in 1892, at the request of the Berlin Court of Appeal, the Leipzig Chamber of Commerce investigated how much moisture wool may contain and recognised 17 % as the norm.
According to the firm, Lambrecht hair hygrometers could withstand heating up to +200 °C, and for drying chambers they were supplied with thermometers of a suitable range. The electric contact version completed this logic: the instrument not only showed that the air had become too dry but also started the humidification itself.
Two eras meet in this instrument. Its basis is an idea from the late 19th century: a bundle of human hair, a scale of auxiliary numbers, a trident at the tip of the pointer and naively confident weather rules that were to be judged fairly only after 30 hours. In the mid-20th century, contacts, PVC-insulated wires and a 24-volt terminal block were built into this mechanism. The "little meteorological observatory" became a component of automation, a link between measurement and control.
In the computing sense of the word, the instrument outputs very little information: at any moment, one bit per limit — "still within range" or "not any more". Yet this was enough to keep humidity in a workshop or a store under control for years, with a relay clicking somewhere on the wall. The network of craquelure on the dial and the oxidation on the pointers show that the instrument spent its life not in a display case but at work, in the very humid environment it was meant to tame. The human hair that Lambrecht once entrusted with measurement was here also given a voice.