THEORY OF DIFFERENTIAL- The sulfur example just cited may illustrate 

 THERMAL ANALYSIS also the difference between an ordinary heat- 



ing curve (fig. 7-1 A) and a differential-thermal 

 curve (fig. 7-1B). Differences in equipment simply involve inclusion of addi- 

 tional material — thermally inert alumina (A1 2 3 ) — in the furnace, and the use 

 of a differential thermocouple. The latter is made from two identical thermo- 

 couples connected by one like wire of each. One thermocouple junction is em- 

 bedded in the sample (sulfur), the other in the inert standard (alumina). The 

 remaining wires then are connected to the pyrometer. As long as the two junc- 

 tions are at the same temperature, induced voltage from each will cancel the 

 other; but if the junction in the sulfur becomes hotter or colder than that in 

 the alumina, the voltage induced in the appropriate direction indicates the dif- 

 ference of temperature between them. A separate regular thermocouple is used 

 to measure the actual temperature of the alumina. 



While steady heating takes place, as in the previous example, readings are 

 made of alumina temperature and of differential temperature. The former is 

 plotted vertically and the latter horizontally on either side of a zero position (fig. 

 7-1B) . In this way, only a vertical line will result as long as the two materials 

 are rising in temperature at the same rate. However, if the sulfur lags behind 

 the alumina during melting, a curve or peak will be traced to the right of the 



A B 



Figure 7-1. A — Heating curve. B — Differential-thermal curve. 



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