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IOWA ACADEMY OF SCIENCE 



position hanging on the right wall. In figure 25 a plan view is 

 shown, with the tops and the hottom of the oven space removed. 

 At the left is shown the electro-magnet device for breaking the 

 current through the heating coils. This magnet is operated by a 



Fig. 2 4. 



current controlled by the device shown in figure 26. This con- 

 sists of a bar of asbestos board, to which is attached a thin metal 

 strip. The gap shown between the silver tipped screw and the 

 silver plug in the metal strip immediately below is 8 mm. This 

 is the amount of rise of the center of the strip for a change of 

 temperature from 20° C to 360° C. 



The oven requires 8 amperes of current to heat it to 450° C, 

 making the cost about 10 cents an hour to operate it at standard 

 electrical rates. The controlling device regulates at 450° C to 



