and Laboratory Methods. '^^^5 



the gas supply to the burners, were adapted by very slight changes to the new 

 requirements. The tubular glass stopper carrying the gas inlet was removed ; 

 in Its place was put a cork, bored to receive one of the copper wires ( IV) from 

 the single-cell battery of the Laclanche type (Samson cell No. 2). The copper 

 wire terminates with No. 'JO platinum wire (Fig. 3, /'A) which is about an inch 

 long; the height of the lower end of this wire above the mercury in the column 

 can be roughly adjusted by moving the copper wire through the cork and wedg- 

 ing it in place by a small wooden wedge occupying with the wire the hole in the 

 cork. The contact of the mercury with the platinum wire serves to close the circuit 

 of the battery, and the other wire from the battery being connected to the mer- 

 cury by means of the screw and piston in the side tube used to adjust the height 

 of the mercury column. Into this single-cell circuit is put a standard 150-ohm 

 telegraphic relay machine of the pattern used by the Western Union Telegraph 

 Company. 



The closure of this single-cell circuit magnetizes the core of the electro- 

 magnet (M.), which, pulling against the delicate spring (S.), overcomes it and 

 moves the armature (A.) away from the post (/'.), thereby breaki7ig the heating 

 current. 



When, owing to the interruption of the heating current, the bath cools, the 

 mercury in the regulator recedes from the platinum point, thus breaking the relay 

 circuit, the electro-magnet becomes demagnetized, and the delicate spring pulls 

 the armature into contact with the post and thus closes the heating current. 

 The finer adjustment of the distance between platinum point and 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 (110 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 parafifine baths of the form shown in Fig. 1 are about nineteen inches in 

 diameter and five 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 street, Cambridge, and 548 Boylston street, Boston. 

 It cost, including the heating coils, between $25 and $30 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 maximum 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 current 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 a 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 regulator, the oxidation may 

 be prevented. 



