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 BAIRD &TATLOCK (LONDON) LTD. 



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Fig. 5. Part of Recorder Chart. 



About full size. 



allowing very accurate readings. Fig. 5 shows a portion of a chart. The great width of the chart giving fine 

 readings, should be noted. Where desired, the recorder can be fitted with an alarm contact to ring a bell whenever 

 the temperature reaches a pre-determined limit. 



Laboratory Apparatus. The simplicity which has proved advantageous in the works is also appreciated 

 in the laboratory, and some useful special outfits and accessories have been devised for experimental work. 



Recalescent Outfit. It is generally known that each grade of steel has a critical temperature above which 

 it must be heated before quenching will harden it. In high carbon steels the passing of this critical temperature 

 in cooling is accompanied by what is known as " recalescence." The steel, as it gives out heat gets cooler until 



Fig. 7. Recalescent Outfit. 



About one-sixth full size. 



the critical point is reached, and then, though it still continues to give out heat, there is a " halt " in the fall 

 of the temperature. 



To find the " recalescent point " is therefore the first step towards laying out a hardening programme. 

 There are several ways of doing this, but the quickest and simplest is with the outfit we have designed for the 

 purpose. 



A small sample of the steel is drilled and slipped on the tip of a small Hoskins Thermo-couple. This couple 

 has its " cold junction " immersed in an oil vessel. A portable indicator having a specially fine scale and pointer 

 is connected to the stem by means of a flexible cable. The sample is now heated by a Bunsen gas flame till its 

 temperature as shown on the indicator is over 850 C., the flame is then removed and the sample allowed to cool. 

 The fall of the reading on the indicator is continuous till the critical or recalescent point is reached when the pointer 

 will " halt " for a few seconds. Steel of this quality must be heated to certain excess above the " halt " point 

 in order that it may be hard when quenched. Usually, for small work, not exceeding half-inch in diameter, 

 30 C. excess over the recalescent point will give good results. Of course, actual trial will be needed to get the 

 best final results. A recorder may be used in place of the indicator, thus automatically recording the " halt " 

 point. For this work the recorder is made with a specially open scale and the chart is revolved more rapidly. 

 Special quotations will be given for outfits for this and other research work. 



CROSS STREET HATTQTST GARDEN, E ; C 



982 



