We have seen how an electric current is generated as a result of chemical action in a cell or battery. We have studied also a second method of generating a current by the use of a generator or dynamo. A third method is as follows:
Demonstration. - Twist together the ends of a small iron wire and a small copper wire and couple the other ends to the terminals of a sensitive galvanometer. Apply the heat of a Bunsen burner to the twisted ends of the wires and a movement of the galvanometer needle will indicate an electric current. Let the heated ends of the wires cool and the needle will return to zero. Hold a piece of ice against the twisted wires and the needle will move in the opposite direction.
The two wires as used in this demonstration form a thermoelectric couple. Such a couple will generate an electric current as long as the twisted ends of the wires differ in temperature from the ends coupled to the galvanometer. An extensive use of this current is made in the thermoelectric pyrometer, which is valuable in various industrial operations.
When this instrument is used for temperatures not to exceed 1800° F. (982° C.) the wires are made of nickel alloys and the couple is called a base metal couple. The wires are welded together at one end and are separately inclosed in porcelain insulating tubes as shown in Fig. 1. When in use they are inclosed in an outer protecting tube.
For high temperatures the thermo-couple wires are one of platinum and the other of platinum alloyed with ten per cent of rhodium. This form can be used for temperatures as high as 2900° F. (1593° C.). High temperature pyrometers are used in measuring the temperature of furnaces, through the wall of which they are inserted.
The reading instrument is a form of millivoltmeter in which the scale is calibrated in degrees Fahrenheit or Centigrade as required. This instrument with its cover. removed to show its interior construction is shown in Fig. 2.
