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Electrical heating of paraffin baths.

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It. h4. STRONG
From the Hull Zoological Laboratory, University
0s Chicago
In thc. autumn of 1910, I took u p the problem of electrical heating for
pnraffin haths in the zoological laboratories of the University of Chicago.
The system clescribed by Mark1 was considered, but certain fcaturcs of
his equipment did not seem t o
suitablc for the Lillie type of bath used
in our 1:d)oratorics. Somc modifications wcre made, and these especially
will be tliwusscd in this paper. Observations on the amount of current
cmployctl 2nd other data have heen included with the hope t h a t they
may be u ' ~ ~ f tuol some readers. It should be noted here, t h a t the system
tlcscribctl in this paper is used with a 110 volts direct current. Changes
would he necessary for an alternating current.
Diifcrcnt kinds of heating units were examined, and three were tried.
The. moit wtjsfactory results have been obtained with so-callcd disc
stoves, such as may be oht,ained on the market. One of these stoves is
rnountcd on :L metal frame, and its surface is in contact with the center
of the p,:traffin bath bottom. The stove and its mounting are within an
a5bestos-lined box of galvnnizecl iron which supports the bath and presumably affords some cconoiny in the use of heat (fig. 1, b ) besides being
:L protcction against fire. A door will be seen in the side of this box,
which givcs ~ ( Y T S S t o the stove inside. The current used by the stove
is controlled by a thermostat, locatcd at the rear, right-hand upper
vorner (fig. 1, t ) which operates a n automatic switch in a case below
(fig. 1, 6 ) . The switch case should be mounted in a more accessible
position than is shown in figure I. This bath will be designated as A ,
:tnd another similarly equipped will be called B.
Various forms of thermostats were considered, and the adnptcd
Keicliert gas-rcgulator clcscribcd by Professor Mark was tried for :I
titne. It was found, however, tlint this requires an unreasonable amount
of :Lttention t o k w p i t in ordcr, and something which mould be less
ily injured w:is tlesirctl. There was no suitable location for a metal
thermostat such :is has been described by
and it also swmcd
pxraEn bath hctttcd by electricity. Am. Xst., vol. 37, no. 434,
1 E. L. RIarh,
1913, pp. 115-119
J. C i . Land, .in elcctrical constant temperaturc apparatus. Bot. Gaz ,
vol. 52, no. 3, Soveinbcr 1911, pp. 391-399, 4 text figurw
Fig. 1 From a photograph of p:Lr:Lffin bath cquippcd with clcctiical heating
apparatus and supported by a galvanized iron box, b. The box is lined with asbestos. An electrical ‘disc stove’ is mounted inside thc box with its surface in contact with the center of the paraffin bath bottom. The top of the thermostat
adjusting screw appears a t t ; s, galvanized iron case containing automatic switch
or ‘cut-out;’ c, cable supplying current to the apparatus; m, thermometer.
desirable t o have a thermostat which would be controlled by the temperature of the water in the bath jacket.
The double branched tube, shown in figure 2, was designed with the
idea of dropping all the glass portion of the apparatus down into the
bath water jacket through the small opening which is provided for gas
regulators. This arrangement was desired in order that as little as
possible of the thermostat should be exposed t o such injuries as might
occur outside of the bath. The result is, that only the square vulcnnite
block and the parts above it (figs. 3 and 4) arc exposed to view and to
chance blows. The tube is suspended from the vulcanite block, just
mentioned, by means of the flanges a t the upper ends of the two
In order to relieve the glass tube from lateral strains, it was found
desirable t o enclose the whole tube in a brass case which was fastened to
the vulcanite block as may be seen in figure 4.
Mercury was poured into the tube until it entered the lower portions
of both branches. Openings in the side of the brass case make i t possible to view the mercury levels in the branches. The height of the mercury in branch B may be varied by an adjusting screw which regulates
the level of the mercury in branch A . It may be set for a wide range of
A platinum needle is suspended in branch B from a slender but stiff
rod which is attached t o an arm (fig. 4, a ) , a t the base of one of the binding posts above (fig. 4, b.p’.). It has been found important to have the
needle point well centered to avoid contact with incrustations which
accumulate gradually on the inside of the tube above the mercury,
sometimes, and which seem t o conduct electricity t o the column of mercury even when it has dropped below the needle point.
Various inside diameters were tried for the tube branches, and the
most satisfactory have been about 6 mm. for branch A and 3 or 4 mm. for
branch B.
The delicacy of the thermostat varies, of course, with the mass of the
mercury and inversely, with the diameter of the mercury column in
branch B. My experience has been that the greatcr the diameter of
this mercury column, the longer may the apparatus be expected to run
without attention. With much smaller diameters, i t is necessary t o
clean branch B rather frequently, i.e., every few weeks. Accumulations
of oxidized mercury near the point of ‘make and break’ in small bore
tubes, cause the thermostat to work erratically. The use of oil to reduce
the arcing and consequent oxidation of mercury only postpones the
coming of trouble, in my experience.
The automatic switch used (figs. 5 and 6) is a mercury cup ‘cut-out’
designed and made by Mr. Julius Pearson, mechanician in the Ryerson
Physical Laboratories a t the University of Chicago. It has mercury
CUPS which are deep enough to prevent mercury from splashing out
when the switch-lever plungers descend or rise. The coil in the electromagnet is wound with a very fine wire, and it offers sufficient resistance
to make a rheostat unnecessary. The cut-out is mounted on an asbestos
hoard base. Upon the advice of an electrician, a piece of fuse wire
(fig.6, f ) W:~S inserted, but the maker regards this as superfluous.
X ciicnper but (to my mind) less effective form of cut-out has been
&>scribed by Land3 which may be substituted for the one dewribed
not. i;az.,
3 W. J . G . I,nnd, An electrical constant temperature apparatus.
vol. 52, no. 5, pp. 391-399.
Big. 2 Glass port.ion of thermostat. X I. A , branch of tube which contains
adjusting screw; B , branch containing platinum point for ‘make and break’ of
circuit. This point may bc about 2 cm. from the top of the branch. The level of
t,hc mercury in tlie other branch should be a little lower. The tube used in t,he
thermostat which is shown in figure 4, had somewhat larger diameters.
Fig. 3 Diagram of vulcariitc block which appears in figurc 4,2’. b. X 4. T h r
11loc:k is cut through its initltllt~for convenience in mounting the glass tube (shown
in fig. 2), which h:tngs by the fl:tngcs at the points indicntcd by the 1:irger circlrs.
The two halves of the block arc held togcther by screws whose hc:ids appear a t tlic
top of the figurc. Tlic positions of binding posts :LIT indic:ttccl by the s m d l e r
Fig. 4 From photogr:iph of thcrmostat complct,c. X :. The gl
in figure 2 is h e x inrlosetl by ii brass protector which is screwed t o the vulcnnitr
plate above. Openings 11~:tr1.hc top permit inspection of the mercury levels.
Branch B of figurc 2 is in view; h.p. and b.p.‘, binding posts; u , arm which supports
slcnder rod with platinum point; s, adjusting screw; c.b, vulcanitc block shown in
figurc 3. The atljiistirig screw 11:~sa closcly-fitt,ing plug at, its lower cnd and :t
small amount of mercury is p1:ic:cd abovc t,he plug as a seal. The scaletl plug prcvents mercury from slipping above tlic end of the screw.
Fig. 5 From photogrnph o f :Iritomat,ic switch of ‘ r u t out’ i n i t s case (fig. 1, s.).
X 1 / 4 5 , The front of tlie casr is closed by a glass door, part of which appears at
thcx left. The door is kept closrtl by :L small padlock.
When electricity is being used by the stove, the switch lever (fig. 6,
I ) is down, and the plungers at its right hand end have their lower ends
immersed in the mercury below. This arrangement completes a circuit for the stove. As the temperature of the bath rises, the mercury in
the thermostat expands until the column in branch R (figs. 2 . 4 and 6, t ) *
rises to the level of the platinum point above and a contact occurs.
Immediately a circuit is made for the thermostat through wires which
join t h c hcnting circuit at s and s', figure 6. The consequence is that
Fig. 6 Diagram of automatic switch and condensed wiring plan for the cntirc,
heating apparatus; p , binding posts for clectric supply; h, stove; t , thermostat ;
V L , electro-magnet; I , switch levcr; J , fuse wire; c , mercury cups. Thc rurrcnt
which passes through the thermostst is shunted from the heating cirruit a t s antl
s'. The mercury in the cups and in the thermostat is shaded black. The diagram
shows thc thermostat in circuit with the current t o the stove broken.
the elcctro-magnet (fig. 6, n2 ) is supplied with currmt, and a piston
whivh is att2clied near the left hnntl end of thc switch lever is pulled
upward. This raises the switch lever, and the plungers :it the riglit
hand end of the lever are withtlrtzwn from thc mercury (as is the case in
fig. 6) with a breaking of the circuit for the stove a h n consquence.
As the bath temperature lowers with the cooling of the stove, the mercury in the thermostat contracts until the platinum point is no longer
in cont:ict with the mercury in branch R. The thermostat circuit
breaks, thcn, antl cuts off the current t o the electro-magnet (fig. 6, m )
whereupon the levers descend and the stove is again placed in circuit.
I n figure 5, a coil spring will be seen above the switch lever and attached
near its middle. This spring takes up some of the weight of the lever and
its plungers, the electro-magnet not being strong enough to do the work
Bath A is 46 cm. high, 33.5 cm. deep, and 80 cm. wide. It is heated
by a 770 Watt stove. A 550 Watt stove is used for bath B which is
51 by 38.5 by 57 cm. in size. Both stoves have been tested in service,
and they were found using almost the exact amounts of current for
which they are rated by the manufacturers. The two stoves were
obtained from different makers.
Observations were made when the bath temperature was 55"C., and
with different room temperatures, of the periods of rest and service,
that is, of use and non-use of electricity by the stove. The stove is
in service to some extent during the period when current is not being
used. However, it furnishes heat to the bath during only a small part
of a rest period. With a room tempcrature of about 21"C., both stoves
were in circuit approximately half the time. For bath B, the periods
were 13 to 13.5 minutes, and they were several minutes shorter for bath
B. When the room temperature was only 16"C., the periods of service
averaged about twice as long as the rests. During the winter, the room
temperature was sometimes below 1O"C., a t night, but the paraffin was
found melted and the apparatus in good order, the following morning.
No actual observations, concerning the ability of the stoves t o maintain the required temperatures when the room temperature was below
15"C., were made.
The number of kilowatt hours for a day of twcnty-four hours, when the
room temperature is about 21°C.) would be, then, for bath A about 9.2,
and bath B would use in the same time about 6.6 kilowatt hours of current. The cost of the heat for baths, thus equipped, can of course be
estimated only by considering the local charge for electricity.
Observations were made for a period of about thirty-five minutes on
the constancy of a thermostat which had a column of mercury in branch
R, 3 mm. in diameter. The room temperature was about 21°C.) and the
bath was running a t 55°C. During this period, there were five changes
in the current which involved two periods of rest for the stove. NO
measurable changes in temperature for the bath were indicated by the
thermometer employed, during this period. Later, a thermometer
with graduations of 0.2"C. was used for a period of about one and onehalf hours, when the difference between the lowest and the highest temperatures was 0 . 6 T . Thc thermostat was, of course, in good running
order a t this time.
The most satisfactory resu!ts have been obtained with the 770 Watt
stove which was bought, of the Simplex Electric Heating Company, and
I am informed that their stoves have given the best results for others.
They are loratcd a t Sydney and Auburn Streets, Cambridge, Mass.,
with ;2 nwtcrn office in Chicago, 1144-1146 Monadnock Block. This
770 Watt, 110 volts stove No. 1704 is listed a t $8.50 with a 25 per cent
discount t o educational institutions. It has been in continuous servicc
for over a year, except for a period of ten weeks during the past spring.
Another stove, used for bath B and of less power, burned out recently aiitl
is being repaired. It is of somewhat different construction and w:i<
bought of another firm.
The glass tube shown in figure 1 was made by W. J . Roehm, 170 West
Randolph Street, Chicago, a t n cost of 63 cents. The mounting of thi.;
tube complete was done by the mechanician alrcady mentioned in thi.
paper for about $4.00, and his charge for the automatic switch appar:ttus
was $i13.00. The cost of both the thermostat nnd the automatic switch
can, of course, be greatly reduced if some of the work on t h ~ mc a n be
donr in the laboratory by tl-ic. in5tructor or an a tant. X chcnper
electro-magnct can be bought in t h r market and the switch may he
adapted to i t with fair results. The box supporting the bath :mtl the
case for the automatic switch wertk made by a local hardwirc firm at :I
small expense.
I n conclusion I acknowlcdgc helpful suggestions received from Prof.
W. L. Tower in the planning of this equipment.
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paraffin, electric, bath, heating
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