An Improvised Electric Thermostat Constant to 0·02°
Samuel Clement Bradford · Biochemical Journal · 1922
IN these days of elaborate physical apparatus it may be of interest to describe a simple thermostat which can be made easily from material at hand and is sufficiently constant for most purposes.The apparatus, shown in the photo- graph, was required for an investigation expected to extend over twelve months or more and needing only moderate constancy.The principle of the stirrer, on which the success of the thermostat depends, was suggested by Mr S. Bradshaw, to whom the author is indebted, also, for much kind assistance in making the apparatus (Plate I).The stirrer comprises a solenoid, A, with an iron plunger, B, mounted on a glass tube carrying an ordinary cork bung, shown in dotted outline, at its lower extremity.The upper part of the glass shaft bears two rubber corks, C and D, that strike the left arm of the brass lever, E, whose right arm carries a cylindrical plumbago contact.As the stirrer reaches the lower end of its stroke, the cork, C, pulls over the lever, causing the plumbago contact to engage with a similar con- tact, F, above.The current from the 110 volt supply now flows through the solenoid, raising the plunger until the cork, D, throws the lever back again, breaking the circuit and allowing the plunger to fall.To render the lever stable in either position, it is made top-heavy by means of the weight, G, carried on a stout wire soldered to the lever near its pivot.For temperatures up to 250, a 16 c.p. carbon lamp is sufficient as a heater.For higher temperatures a more powerful lamp may be employed, or an auxiliary one continually alight.The brass parts of the lamp are protected from the water by a short piece of 1 inch rubber tubing.As pointed out by Lewis [1922], the heater should be placed near the thermo-regulator, M, which is similar to an ordinary toluene thermo-regulator for gas.This was made from a stout glass test-tube of about 50 cc.capacity when sealed to about 3 inches of 4 mm.glass tubing.To this was joined about 6 inches of 1 mm.bore capillary tube, having a short wider tube at its open end.About 2 inches from the upper end of the capillary, a short piece of 4 mm.tube, N, was sealed at right angles.The wider tube, joining the capillary and bulb, was then bent through 1800 near its junction with the bulb.The bulb contains toluene and some mercury which also fills the Bioch.m