336 
Journal of Agricultural Research 
Vol. V, No. 8 
of the sides, for example, there is an area of about 5.8 square meters. 
The total resistance of the wire in the space is 143 ohms. In series with 
this wire but exterior to the space is a resistance unit that may be varied 
according to the need for current. If a unit of 200 ohms' resistance were 
used, there would be a little over 0.64 ampere of current flowing in the 
heating wire, the pressure of the current being 220 volts; and the total 
amount of electrical energy (PR) dissipated in the 143 ohms of wire 
would be nearly 59 watts, or roughly 10 watts per square meter of sur¬ 
face of zinc. Similarly, the area of the lower zone is about 8.9 square 
meters, and the resistance of the wire in it is 195 ohms; with an exterior 
unit of about 125 ohms in series with the heating wire, the amount of 
energy dissipated in the latter would be about 92 watts, or slightly over 
10 watts per square meter. There are close to 2.9 square meters in the 
top section and the same area in the bottom, and in each of these sections 
is a heating coil of 117 ohms; with an exterior unit of 325 ohms in series 
with it, the current in each heater would approximate 0.5 ampere, and 
about 29 watts would be dissipated in the 117 ohms of resistance wire, 
or 10 watts per square meter. 
In controlling the temperature of the zinc wall cold water is kept 
flowing continuously through the brass pipe in the air space outside of 
it at such a rate of flow, depending upon the temperature of the water, 
that the temperature of the unheated air would be lower than that at 
which the wall is to be kept. With a constant flow of water the tem¬ 
perature gradient along the pipe is quite flat in comparison with what it 
would be if the rate of flow were increased or decreased as the air would 
need to be cooled or heated; in other words, the cooling effect is fairly 
uniform throughout the length of the pipe. At the same time electric 
energy is converted into heat in the resistance wire until the air is warmed 
enough to bring the wall to the desired temperature. Since this dis¬ 
sipation of heat is equal in all parts of the wire, the total mass of air in 
the space is quite uniformly heated. Under these conditions to change 
the temperature of the wall requires only an increase or decrease of the 
current in the resistance wire, according to whether the wall is to be 
heated or cooled, which involves merely the adjustment of a rheostat in 
series with the wire, so that regulation is easily and quickly effected. A 
rheostat of oxidized constantan wire wound on an enameled metal tube 
and having a sliding contact passing over successive turns of the wire, 
with a resistance of about 980 ohms and a current-carrying capacity of 
1 ampere, is in series with the resistance wire comprising the heating 
coil in each section. The four rheostats for the different air sections to 
be controlled are attached vertically to an asbestos slab at one end of 
the observer's table, as seen in Plate XXXVI, figure 1, with the slid¬ 
ing contacts in easy reach of the operator reading the galvanometer 
deflections. 
