resister yet it will not fall down. A iurnace was built with plates having these dimensions: i.enefh nt t o n . . . . . . . . ~. ,405 mm 16 ieclles Leeyfh at hutinm . . . ~. . . . . , 2 5 5 mm. 1" inclws ~
\!'idfh.. . . . . . . . . . . . . ~. . . E65
81mi.
6 S incllei
The two sections of the resister contained 7 1 plates cach. This when cold hag a resistance oi o . 2 0 0 ohm and when running at the full capacity of r j o kilowatts. and with a temperatute in the furnace oi 1 4 0 0 ~ C . , the resistancc was 0 . 0 3 7 j ohm. This resistance is due almost a l t w gether t o the contact resistance between the plates, ior by calculating the resistance o f the carbon itself we find t h a t it would not amount to morc than 0.00064 uhm. In unier to rcgulate the rate uf genera!.ion o f enctrgy in the rcsistm there must be some means oi varying the voltage a t the terminals oi the iurnace. A t Hohcnlohehntte, as well as in thc FitzCera.li1 and Bennic Laboratories where these furnaces have been worked, this is done b y means oi a transformer with sevcral taps brought out from the primary coils which allow the voltage on t,he secondary circuit to he varied from ,io to roo volts in 2 . j - v o l t steps. and irom 1 0 0 to 2 0 0 volts in j-volt stel's. n t h a t t h c weakest i'art in this iuvnacc is the carbon resistcr. due t,ii thc iact t h a t ii uwrkinz in an oxidizing atmosphere the resister will be dcst,royed. In t h e particular work ior which i t was dcsigned, howrver, there would tic, n o d;inger of t h i s because the furnace i s tilled with vaimr of met,allic zinc. Iluring the pro s oi hcating tht, furnace. 0 1 at a n y time when zinc vapors were not generated. t h e w would he danger oi burning through air leaking i n ; b u t this is easily ovcrcoine b y keeping a reducing
I he furnace is a highly efficicmt one. I n one of the earlicr models whcni the heat insulation was far irom being satisfactory careiul determination of all heat were madc. When working at tempcratures bctween i z joo and 1 ~ 6 0 'C. the total heat losses werc 3 3 kilowatts, and when working at temperatures between rqoo' and r 4 j o o C. the heat losses were 4 2 ki1owat.ts. Consequently when the iurnace is working at iull caI n c i t y , r g o kilowatts. the thermal efficicncy a t 1 2 j o O is j.X jler cent. and a t ~ 4 2 j "C. is 7 2 per cent. K o exact determinations of the efiiciency of later modcls have becn made, but it is k n o v n t o be much highcr than those given above. I.
I Fii: 5.
Thc mct:iIliirgic:al cnil oi the lrobliwi has not hecm ci,mpletely worked out. but thc satisfactory working oi the furnace has bccn clcarly ilcmonstrated. anti iurnaors built on similar imnciplcs have been used exix.riment;illy wit,h great s u ~ c e s sin the mclting of aluminum, c q > p e r . brass, c t c This is thought to be of some interest, as a dcvclojrment in the use oi electric furnaccs using the heat generated by thc passage ( i f an elcctric current through a resister. There is a tendency in electric furnace work to employ the arc which is d t e n a mist,nkc because of the difiiculty in regulating the tem])eratiire. Finnlly, the furnace describcd above from its construction lcnds itseli rcadily t o adaptations which pcrmit of using the comh i n d heat effects of fuel and electricity, and i t is thought t h a t ii great i ~ i t u r fis in store for furnaces oi that t y p e
A RADIATOR FOR PLATINUM CRUCIBLES. I i Y \v. >I ' i ' I I < I K V I O N . I n Kl'ccivrd h l U i C l , 16. I R I I
atmosphere in t h e furnace slightly abovc external pressure. I t has been found by actual experiment t h a t a iurnacc ai this type running continuously ior two months showed no appreciable wear of the resistcr. The regulation oi temperature in this furnace is most sat,islactory. I n the IIohenlohehutte experiments thermo-couples of pyrometers were placed in several 1)art.s of the furnace t o study the temperature conditions carefully. I t was iound t h a t the most accurate regulation o f the tempcrat,ure in the furnace nas 1x1ssible, the workman in chargc adjusting the rate oi generation of energy in the resister so as to keq' the needle oi the pyrometer stationary.
The accompanying drawing illustrates a very simple radiator ior the evaporation oi solutions t o dryness in platinum cruciblrs. 1' is the platinum crucible, held in position by thc Hoskins chemists' triangle M (a special nickel alloy). The ends oi the Hoskins' triangle are cut off at a point about midway their length and bent upw-ards. T h c I-loskins triangle is supliortetl b y the walls oi the nickt.1 crucible N at a height. t h a t will hold the platinum crucible with its bottom and sides about equid.stant from the bottom and sides of the nickel crucible. The outer c.rucible is siipported by a triangle of nickel mire, I
