However, let us not trouble ourselves with too many definitions at one time, but instead amuse ourselves for a while by running through a little experiment with a piece of carbon tool steel similar to that which we have just been discussing. For our investigation we will also need a special kind of thermometer for measuring high temperatures. Such an instrument is known as a “pyrometer”. Now we will drill a little hole in the test piece of carbon steel and after inserting the “couple” of the pyrometer into it, place the same in the electric furnace. As the current is turned on, the test piece begins to grow warm and then hotter and hotter, gradually up through a range of temperatures which are continually recorded by the needle of the pyrometer. 800, 900, 1000, 1200 degrees Fahrenheit are uniformly reached, and the temperature of our test piece continues to rise, as the absorption of heat progresses. Suddenly, however, the test piece assumes a bright glow and the needle of the pyrometer ceases to advance, and we note that it is pausing at about 1350 degrees Fahrenheit. Then after its pause, the advance is again resumed until the piece has become almost ready to melt. By plotting the uniform periods of time at which we read the different temperatures recorded by the needle of the pyrometer, against the temperatures as read, we would have a picture of our phenomenon something as follows: [Illustration: Graph showing the course of the temperature curve as a function of the heating time of the metal sample.] Now let us begin to let our test piece cool off gradually. The temperature of the furnace is lowered and the uniform range of cooling temperatures is recorded by the ever sensitive needle of the pyrometer. Suddenly as before, the test piece assumes the brilliant glow noted previously, and again the needle comes to rest, but this time we note that the recorded temperature is about 1250 degrees Fahrenheit instead of 1350 degrees Fahrenheit as before. Evidently there has been a certain tardiness or “lag” which has caused the phenomenon to take place a little too high going up and a little too low coming down, and in fact the metallurgists tell us that such is exactly the case, and that the real point in which we are interested lies just half way between the two points indicated, as we shall presently see. If we again represent the results of our latest experiment graphically, we would have a picture something as Fig. 2. [Illustration: Graph of the cooling curve of the metal sample over time] Now placing the second curve so obtained on the first, we are able to study the following interesting relationship. Fig. 3. [Illustration: Graph combining the heating- and cooling-curves from before and demonstrating the critical range]