1913| LAND—CONSTANT TEMPERATURE APPARATUS 305 
adds slightly to the cost, but will permit the sediment which appears 
in water-jacketed baths to be removed easily. Great care must 
be taken to have all joints absolutely water tight, since a leak 
will cause the destruction of the resistance wire in a few hours. 
An inspection of the diagram (fig. 2) shows the arrangement 
is such that the resistance coil is completely surrounded by water 
except at the.lower end, thus insuring rapid conduction of heat. 
Also the coil can be removed easily if repairs are ever necessary. 
- 3.—Diagram of automatic switch; p, p’, binding posts to connect with 
Source a electrical supply; e, op ta a, armature and switch; r, gasket for 
lamp rheostat; #, ¢’, binding to be connected with thermostat; ¢c, container for 
mercury (m, m); ki; Se binding et to be connected with heating coil. 
In two years’ use, only one has needed repairs, made necessary by 
spilling water on the head of the coil when the water jacket was 
being refilled, 
The automatic switch (fig. 3) is a slate base having an electro- 
magnet (e), armature (a) with switch, and a lamp rheostat (7) at 
one end, and the slate cups (c) filled with mercury at the other. A 
convenient size for the slate base is 20X11.25 cm. The magnets, 
which can be had very cheaply from electrical supply houses, should 
have a resistance of about 20 ohms. The armature (a) should be 
fastened by a spring to a pillar rigidly attached to the baseboard, 
