A BATH THERMOSTAT
In every laboratory there are times when a constant-temperature water-bath is very necessary for the success of some experiment or other. Such a piece of apparatus, however, is expensive when purchased from laboratory supply houses. The result is that many laboratories are not so equipped and exercises requiring a closely regulated temperature are either poorly performed or omitted entirely. The writer, needing such an instrument, devised the bath shown in the accompanying diagram. No originality of principle is claimed. It is inexpensive, the exact cost being
determined by the number of parts already on hand. While the diagram is largely self-explanatory, a few words may serve to make the operation of the bath somewhat easier to understand. The lamps, L, heat the water in the ten-gallon stone jar, D, and are connected in series with the armature of the telegraph sounder, T. A central wooden strip extends down into the water from the wooden frame, C. This strip carries the thermostat which is made as follows: a widemouth gas bottle, E, capacity, 150-200 cc., is fitted with a two-hole stopper carrying the glass tubes, A and B. Mercury is placed in the bottle to a depth of one-half inch. Tube A has a platinum wire sealed into the end to make contact with the mercury in the bottle. Its exterior portion is also partly filled with mercury to make contact with a lead from the dry 969
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JOURNAL OF CHEMICAL EDUCATION
MAY,1931
cell. Tube B is open a t both ends and is thus also partly filled with mercury, depending upon the pressure in the bottle. The electromagnet of the telegraph sounder is connected to a dry cell and the thermostat as shown in the diagram. The adjustable wire extending down into tube B may be held a t any desired position by making a few crimps in it. S is a mechanical stirrer; any available stirrer may be used. The operation of the thermostat is as follows: When the water begins to heat, the confined air in the bottle expands, driving the mercury up tube B, finally making contact with the wire. This closes the circuit containing the dry cell and the magnet. The latter pulls the armature of the sounder down, breaking the lamp circuit a t the point P. The water begins to cool, but before appreciable cooling takes place the contraction of the air in the bottle causes the contact in tube B to be broken, allowing the lamp circuit to close again. By making B of a small-bore tubing, the temperature differential to cause the cutting on and off of the lamps is only a few tenths of a degree. This is, of course, a measure of the accuracy of the instrument. For different temperatures, the wire in tube B may be adjusted to different levels. Lamps of various power may be used for different heat ranges. Seventy-five-watt lamps are convenient for the range from room temperature to 40-45°C.
