SCIENCE AND INDUSTRY. 
temperatures. The problem is one of making a delicate instrument 
pointer complete an electrical circuit, which, in turn, has sufficient 
power to operate switches and valves. : 
The problem is a comparatively easy one’ for low! temperatures, 
but its difficulty increases with the higher temperatures, where a thermo- 
electric pyrometer must be used. A: thermo-electric pyrometer consists 
of a thermo-couple formed of two dissimilar wires with the wires joined 
at one end. When the junction of the two dissimilar metals is heated 
it produces a small electro-motive force, depending upon the tempera- 
ture. The electro-motive force can be used to actuate a millivoltmeter, 
Bi is nothing more than a voltmeter graduated in thousandths of 
a volt. i 
For temperatures up to 2,000 degs. Fahr., it is common practice to 
use a thermo-couple formed of base metals, one wire being nickel 
chromium and .other nickel aluminium. , Such a thermo-couple 
produces 40-1,000ths of a volt, or 40 millivolts, for a temperature of 
2,000 degs. Fahr. For higher temperatures up to 3,000 degs. Fahr., 
platinum wires must be used for the thermo-couple, and it is common 
practice to use one wire of platinum-rhodium, the other chemically 
“pure platinum; such a thermo-couple produces only 20 millivolts, or 
20-1,000ths. of a volt, for a temperature of 3,000 degs. Fahr. 
One can imagine that, with these very feeble currents—thousandths of 
a millivolt—it is impossible to bring a pointer against contact devices to 
actuate a.switch or valve. The pointer on an instrument of this 
character can be readily blown across the scale by the slightest current 
‘of air, unless protected by its housing. ‘To cause such a delicate pointer 
to perform operations requiring much power, it has been found neces- 
sary to depress this pointer on contact-making devices which, in turn, 
‘ean complete the circuit to operate ‘the switches. The pointer is 
_ depressed at intervals usually of one every half-minute or minute. 
There is a central or neutral zone, and on each side of this a contact 
which represents the low and high sides. 
Let us assume that we are maintaining a temperature of 2,400 degs. 
Fahr., and that we wish to maintain this within 20 degs., plus or minus. 
As soon as the temperature has risen to 2,420 degs: Fahr., the pointer 
when depressed on the contact device will open the circuit; likewise, 
when the temperature falls to 2,880 degs. Fahr., the pointer will be 
depressed on the low contact, which, in turn, will close the circuit. The 
‘contacts, which are made and broken when the pointer is depressed on 
the contact table, are tungsten tipped. “The total current passing 
through these contacts as a relay is used does not exceed 50 milliamperes. 
The relay is actuated by the opening and closing of the contact, and 
the relay in turn operates a solenoid, which causes the switch mechanism 
to open and close. Switching mechanism is, of course, required for 
electric furnaces. In place of the switching mechanism we can cause 
the solenoid to operate a valve for the control of gas or oil. 
_ The rapid increase in the number of electrically-heated furnaces a nd 
_ovens facilitates the ease of handling temperature control, for, without 
question, the automatic control of an electric furnace offers fewer 
problems than where oil, gas, or coal must be controlled. Furthermore, 
the extensive adoption of pulverized fuel for firing also increases the 
possibility of automatic temperature control, which is not practicable 
where coal itself is used. . 
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