1911] LAND—CONSTANT TEMPERATURE APPARATUS 393 
1.5 mm., or the thermostat will respond unpleasantly to any tremor 
of the bath or even in the laboratory building. If desired, brass or 
aluminium may be substituted for the zinc. The regulating screw 
should be made long enough to provide for quite a range of tempera- 
ture. When the thermostat has been adjusted to the required 
temperature, it will need no further attention, except perhaps to 
brush the dust from the contact points at very long intervals. 
To raise the temperature turn the screw to the right, to lower it 
turn it to the left. 
The heater (fig. 2) is a water-jacketed resistance coil of brass 
tubing and German-silver resistance wire. The tubing need not 
be thicker than 1 mm. The coil should be proportioned to the 
size of the oven it is intended to heat. .For ovens having internal 
dimensions of 20X 2526.25 cm., and for temperatures of 30°- 
PC., the size given here has been found suitable. Such a coil, 
however, will heat much larger ovens satisfactorily. The brass 
tubes should be about 15 cm. long. The resistance coil is four 
layers of no. 21 German-silver wire, wound on a tube 3 cm. inside 
diameter. The layers of wire are carefully insulated from the 
tube and from each other with asbestos paper about 0.6 mm. 
thick. The wire is wound with an engine lathe 24 turns to the 
inch under considerable tension, and the ends are brought out to 
binding posts (s, s’) in the slate head of the coil. Wound as 
described, the current at r10 volts measures about 2.2 amperes. 
The water jacket is made of 3 concentric brass tubes, the outer 
one being 6.25 cm. in diameter, the middle one 4.5 cm., and the 
inner one 3 cm. outside diameter, so that the tube of the heating 
coil will slip over it in close contact. The inner tube is closed at 
the lower end with a brass disk soldered tightly in place. The 
upper end remains open, and is fastened to the middle tube by a 
brass ring. The lower end of the middle tube is in turn fastened 
in'a similar manner to the bottom of the outer tube. A hole is cut 
in the outer wall of the bottom of the oven, and the outer tube 
soldered directly to the bottom. If preferred, the outside tube 
may be threaded and screwed into a flange soldered to the bottom 
of the oven. If this method is used, a rubber gasket should be 
Placed between the flanges, as shown in fig. 2. This arrangement 
