IsHAW] A SENSITIVE BATH-THERMOSTAT 133 



tested readily with an electrical thermometer or with one of the 

 Beckmann type. 



(f) The stands for objects to be immersed in the bath can be 

 constructed according to the requirements of the user, and attached 

 without difficulty to the inner tub. It is necessary that they should 

 be perforated or latticed, in order that no appreciable effect may be 

 made upon the circulation of the oil. 



3. Notes on the Regulation. An examination of the following 

 notes on the theory and adjustment, together with the sample numerical 

 data, should make the use of the apparatus, or the slight modification 

 of its design without loss of sensibility, an easy matter; and the 

 "trial and error" preliminary tests which are usually tedious, should 

 mostly be rendered unnecessary. 



Let Hi be the average heat loss in calories per second of the 

 liquid in the bath when it is operating at T°C. 

 " Ho be the heat in calories per second supplied continually. 

 " H3 be the additional heat in calories per second supplied 



intermittently for regulation. 

 " Pi be the period in seconds of rising temperature while 



H3 is being supplied. 

 " P2 be the period in seconds of falling temperature while 

 H3 is switched off. 

 It is apparent that the heat loss during p2 must have been supplied 

 'during pi, therefore, very approximately, we have 



(Hi - H2)p2 = (H2 + H3 -'Hi)pi 

 which, arranged more conveniently, becomes 



(Hi - Ho) P2 = ^\^' H3 (n 



Pi + P2 



either side being equivalent to the total heat loss during 0-2, or the 

 total gain during pi. 



The temperature change or variation during the regulation, 

 assuming satisfactory circulation due to the stirring, can thus be 

 determined by dividing either side of this equation by the product 

 MS where M is the mass in grams of the liquid used and S is its specitir 

 heat. Since Hi cannot easily be determined directly the right hand 

 side of equation (1) should be used. H3 can be calculated from the 

 following known quantities, Ri, R2, R3 and V, where 



Ri is the equivalent resistance of the heating coils in the bath ; 



R2 is the resistance of the lamps (or other resistances) outside 

 the bath and always in series with Ri ; 



R3 is the resistance of the lamps (or other resistances) intermit- 

 tently thrown in parallel with Ro for regulation; and 



V is the voltage of the electric mains. 



