396 BOTANICAL GAZETTE [NOVEMBER 
and should extend about 15 cm. to form the arm of the switch. 
Care must be taken not to have the spring too strong, or the arma- 
ture will not be pulled down when the current flows through the 
coils of the magnet. The spring may be much weaker if an 8 or 4 
candle power lamp is used instead of a 16 candle power lamp 
theostat. One wire connects the post p directly to ¢; the other 
post p’ is connected through the electromagnet ¢ and the lamp 
rheostat socket r with ?’. 
_ The switch end of the armature should have two iron pins 
extending downward at a right angle, sharpened, and tinned to 
insure good contact with the mercury in the cups c. In practice 
it is advisable to drill two holes in the switch arm, tap them, and use 
iron machine screws for contact points. The screws should be 
provided with lock-nuts to hold them at the proper length. Switch 
points gradually wear away, due to the small arc which always 
occurs when contact with the mercury is broken. The machine 
screw device permits compensation for this wear. The screws 
should be sharpened of course, to minimize splashing, and tinned. 
The double cup (c) for holding the mercury (m, m) should be 
made of slate. A convenient sized block is 5X3.75X2-5 cm. The 
holes for mercury should have a diameter not less than 1.25 CM™., 
preferably 2 cm., and the edges should be chamfered. The mercury 
oxidizes somewhat rapidly, and in consequence the volume should 
be rather large. Oxidation of the mercury is the chief defect of 
the apparatus, but no way has yet been found to prevent it. To 
compensate for oxidation, a small quantity of mercury must be 
added occasionally. One wire conriects the post p with one of the 
mercury cups, the other cup is connected with #. The post p’ is 
connected directly to h’. 
The switch (fig. 3) may be put in any convenient place, prefer- 
ably out of reach of inquisitive persons. Attach the heating coil 
(fig. 2) to the oven and place the thermostat (fig. 1) inside the ores 
on the upper ledges and connect the posts p, p’ of the switch with 
any convenient lamp socket, fill the cups c with mercury, screw a 
lamp in the socket 7, connect the posts ¢, ¢’ with 6, b’ of the thermo- 
stat. Connect h, h’ (fig. 3) with s, s’ (fig. 2) of the heating coil. 
These connections are shown in fig. 4,a photograph of the apparatus. 
