Signedunsigned
FunctionThis is a box with two instruments that share some of their parts, and are both used to indirectly determine the humidity of air. Separate descriptions for each of them follow:
In the dew point hygrometer, the idea is to gradually lower the temperature of a metal surface until water starts to condense on it, which marks the dew point of the air at that moment. This is accomplished by the fact that volatile liquids like ether lower their temperature when evaporating. One would fill one of the brass-bottomed tubes with ether and submerge in it one end of the glass tube that extends through the stopper. A thermometer would also be placed through the stopper with its bulb immersed in the ether. Since the glass tubes are connected to the brass stand, when one connects the bottom neck of the stand to an aspirator, air enters the ether vessel and helps to evaporate the ether, its temperature decreasing as a consequence. This process would continue until some water condensed on the outside of the brass bottom, at which moment the temperature of the ether would be registered. This would be the dew point. The second tube is for registering the actual air temperature, for which purpose a second thermometer could be inserted into the empty tube. The reading would not be much different to that of any thermometer in the vicinity of the device, and for this reason it was probably rarely used and the second thermometer became unnecessary. One can determine air humidity from air temperature and dew point using a table.
The wet/dry bulb hydrometer is based on the principle that in dry air water will evaporate, and when doing so decrease in temperature. For a particular air temperature and air humidity, there is an equilibrium temperature that a wet cloth will reach, which can be determined by wrapping a thermometer in it. (Hence wet bulb temperature.) In this particular device, the ring stands hold a water-filled vessel that would have a wick immersed in it and coming out of its top to be wrapped around the bulb of one of the two thermometers that would be held from the top of the stand. After several minutes equilibrium was reached, one could record the temperatures of both thermometers (dry bulb and wet bulb) and from these one could determine the humidity of the air using a table. As with the other assemblage, the second thermometer (dry bulb) needs not be mounted on the device and it was probably rarely used as any thermometer in the vicinity would suffice.
Related WorksA one-piece dew-point hygrometer is described in Educational Apparatus and Appliances for Physics and Chemistry Edited by Koehler and Volckmar A.-G. & Co, Leipzig, 1931, p.295-6.