and Laboratory Methods. 



2447 



To a common electric light plug (A) was attached several feet of electric 

 light cord. Some distance from the plug, the insulation was removed from the 

 two cords and two other cords were soldered to them. This provided us 

 with two circuits from the same plug. The first circuit carries the current for 

 the lamp (B). This 32 C. P. lamp we found to be more than sufficient to heat 

 our little bath. The lamp is controlled by the make and break (C) which can 

 be made to break the circuit by a slight movement to the right. The other cir- 

 cuit from the plug (A) passes through the magnet (E) and the thermostat (H). 

 The two electrodes of this circuit are represented by the column of mercury in 

 the glass tube (d) and the soft iron wire (e) projecting down into the tube. 



When the current is first turned on, the lamp (B) which is placed beneath 

 the bath to be heated raises the temperature of the bath. The lower part of the 



Fig. 2. — Diagram to show thermostat as connected for heating. A., electric light plug inserted 

 in socket; B., electric light used for heating bath ; C, make-and-break ; D., post; E., 

 electro magnet; G., lamp used as a resistance; H., thermostat ; a., spring; b., silver 

 contacts ; d., glass tube containing mercury column ; e., soft iron wire. 



thermostat (H) rests in the bath. As the temperature rises, the mercury 

 also rises in the glass tube and comes in contact with the iron wire closing 

 the circuit through the magnet (E) which, pulling against the spring (a), 

 overcomes its tension and moves the make and break (C) away from the post 

 (D), thus breaking the heating current. As soon as the temperature falls enough 

 to break the circuit through the thermostat, the make and break returns to its 

 former position and the lamp (B) once more starts to heat the bath. 



Fearing that an arc would be formed and that even rather heavy contacts 

 might be rapidly fused, we immersed the contacts (b) in a cup of kerosene. 

 This arrangement proved quite satisfactory. The oil acting as an insulator 



