: 
\ V 
| 
LI. An Electric Thermostat. 
By Horact Darwin, V.A., FLRS.* 
[Plate XXIV.] 
HIS thermostat was designed and first constructed as an 
adjunct to the ples | of the 24-inch Refractor 
of the Royal Observatory, Cape of Good Hope. 
In this case the special object in view is to maintain the 
prisms and other parts as accurately as possible at a constant 
temperature, day and night, over a considerable period ; the 
same devices have since been employed in an apparatus con- 
structed for Lord Berkeley, by whose kind permission the 
‘thermostat and its adjuncts are now exhibited. 
The description which follows relates more particularly to 
this latter instrument. 
The general arrangement of the various parts is indicated 
diagrammatically in fig. 1(Pl. XXIV.). -A is an oil-bath, 
well lagged outside, whose temperature has to be maintained as 
accurately as possible constant at any desired temperature 
between 10° C. and 80° C. ‘Three vertical tubes A, A, A, 
are so connected to the bath A that by means of a fan kept 
rotating within Ay, a constant circulation of oil is maintained. 
The oil from the bath enters the tube A, near the top, and 
passes down A,, up Ay, down Az, and back into the bath. 
Two heating-coils are provided, and one of these, which 
is placed in the tube A,, has a current uninterruptedly 
flowing through it from the battery of accumulators Q. 
This may conveniently be termed the permanent heating-coil, 
the circuit which includes it being called the main circuit. 
The other heating-coil forms a shunt across the terminals of 
the battery ; it is only traversed intermittently by a current, 
and is called the intermittent heating-coil, the circuit including 
it being referred to as the shunt-cireuit. By suitably con- 
trolling the current passing through the heating-coils, the 
oil-bath may be maintained at a very nearly constant tem- 
perature. For the sake of simplicity we will assume hence- 
forward that the temperature is to be maintained constant at 
25° C. 
There is a further set of four resistances, two of copper 
and two of manganin, arranged as a Wheatstone bridge B, 
and so adjusted that the bridge is only balanced at 25° C. 
This bridge may be called the controlling-bridge, since it is 
through its deviations from balance and the consequent 
deflexions of a galvanometer, that the supply of current to 
the heating- -coils is controlled. 
[ 408 ] 
* Communicated by the Physical Society : read November 27. 1903. 
