No. 434] PARAFFINE BATH. I 19 



mercury in the regulator is effected, as in gas regulation, by the 

 screw and piston working on the mercury in the horizontal arm 

 of the regulator. 



The points on armature and post at which the heating current 

 <i 10 volts alternating) is made or broken require to be made of 

 heavy platinum wire (No. 15), for otherwise there is danger that 

 the metal will fuse. 



The paraffine baths of the form shown in Figure 1 are about 

 19 inches in diameter and 5 inches deep. The current required 

 to heat such a bath is approximately equal to that of four 16- 

 candle-power lamps. This could be much reduced by enclosing 

 the bath in a glazed frame, one side of which would have to be 

 movable to permit access to the cups. 



The whole apparatus, except the heating coil, was furnished 

 and installed by Clark & Mills, 23 Church St., Cambridge, and 

 543 Boylston St., Boston. It cost, including the heating coils, 

 between $25.00 and $30.00 for each bath. 



In determining the resistance to be used in the heating coil, 

 one should make it as low as possible consistent with the maxi- 

 mum heating capacity required ; for with greater heating capacity 

 the heating current will be in operation a shorter time than with 

 less heating capacity, and consequently the relay current — 

 which is in operation whenever the heating current is not — will 

 be required to work longer, and therefore the battery will become 

 exhausted sooner. When the battery is so far exhausted that it 

 will no longer magnetize the core sufficiently to overcome the 

 spring and break the current, the regulation fails and too high a 

 temperature results. However, a single cell will, with proper 

 care, last for several months without renewal. To guard against 

 the danger of too weak a relay current, one should test the cur- 

 rent from time to time with a voltmeter. 



There is one difficulty with this mercurial regulator, due to 

 the oxidation of the mercury at the time of making and breaking 

 the relay current. This in time causes a deposit on the platinum 

 wire of oxide, which acts as an insulator and thus prevents sharp 

 contact. The deposit may be removed, however, by immersing 

 the platinum wire from time to time in weak nitric acid, and if a 

 layer of high-test kerosene oil covers the mercury in the regu- 

 lator the oxidation may be prevented. 



