THERMOSTATS FOR INCUBATORS. 



73 



extent — but their radical defect is, that when 

 the circuit is completed it is completed : there 

 is no gradation about it. We have seen many 

 such devices, but this defect affects them all. 



After Mr. Hearson's patent was published, 

 mercury was much used in combination with a 

 portion of compensated ether, to be vaporised 

 as in his capsule ; and though all these are 

 practically now abandoned by manufacturers, they 

 are so easily made up by amateurs, or may be 

 so useful in an unforeseen emergency to anyone 

 who can work a little in glass, that it may be 

 well to describe the three principal types. In 

 the J-tube form (Fig. 39) a small bubble of air 

 B and a portion of ether E were introduced into 

 the shorter and sealed arm of a J-tube, the rest 



Fig. 39- 



Fig. 40. 



Fig. 41. 



being filled with mercury to the point F, where 

 a float carried the rod K. At the proper tem- 

 perature the vapour depressed the mercury to 

 about the point A, raising the rod R. There were 

 also several on the balance principle, of which 

 Fig. 40 is a type, the tube here being bent into 

 three-fourths of a circle and balanced on the disc 

 D, to which the rod R is connected. The ether 

 E is at the sealed end as before, and as the 

 mercury is pushed round the tube it causes the 

 disc D to revolve. In another form (Fig. 41), 

 the sealed tube itself is made the float, being 

 inverted in an outer tube or vessel also con- 

 taining mercury. The bubble B and ether E are 

 as before ; the e.xpanding vapour depresses the 

 mercury M, and raises the inner tube, which bears 

 the rod R. As already hinted, all these have 

 generally disappeared, and there is no question 

 that, of all thermostats depending upon the 

 vaporisation of compensated ether, Mr. Hear- 

 son's capsule is far the best. Its simplicity, 

 permanence, and the definiteness cf its zero- 

 point under the same conditions, all make it 

 superior to mercury forms ; and since the 

 expiry of the patent it has been adopted by 

 most English makers, and is made and sold, 



like thermometers, for supply to manufacturers 

 generally. 



Nevertheless, this regulator (with all which 

 depend upon vaporisation of a liquid) has one 

 serious defect, which makes it quite unsafe to 

 depend upon its automatic action alone. The 

 point at which vapour is formed differs with 

 the atmospheric pressure, to the extent in 

 our compensated ether of about two degrees 

 Fahrenheit for every inch of the barometer, or 

 one thousand feet of altitude. As in England 

 the barometer often varies to the extent of an 

 inch and a half, less frequently to within two 

 inches, and on rare occasions even more, it 

 follows that the capsule may vary the regulation, 

 from this cause alone and independent of the 

 teinperature in the drawer, by as much as three 

 or four degrees. 



Metallic thermostatic bars, when really effi- 

 cient, act well and are free from this defect; but 

 many have not been efficient. If two bars of 

 different metals are fastened together side by 

 side, and one metal expands more than the 

 other, the double bar must, when heated, be 

 forced into a curve, with the more expansible 

 metal outwards ; then if one end be rigidly fi.xed, 

 the other will move, and may be used to work 

 the regulator. This has been the most general 

 construction; some wind the double bar into a 

 spiral, which winds and unwinds as the tem- 

 perature varies: Christy's incubator has a spiral 

 thermostat of this kind. Ebonite has also 

 been used as one of the components, and 

 acts strongly, but is to be avoided because it 

 gradually " perishes." Metallic thermostatic 

 bars have been almost neglected in England, 

 chiefly on the ground of the metals rusting. This 

 objection is of course more serious in machines 

 where copious moisture is used, and has been 

 less felt where it is abandoned or used sparingly; 

 but even in a moist chamber the difficulty is 

 easily overcome. The Prairie State Incubator 

 Co. surmounted this real trouble entirely in the 

 case of their thermostats, which are composed 

 of iron and hard brass (perhaps the most sus- 

 ceptible to rust of all metals), by tinning both 

 metals separately before they are put together, 

 and dipping the bars again into a bath of 

 melted tin after they are riveted together. 

 American patents are numerous ; but we have 

 not found very much of note, and will content 

 ourselves with showing, in Fig. 42, that used 

 in the now well-known " Cyphers " incubator. 

 Each thermostatic bar F, of which there are two, 

 is about twenty-four inches long (or the whole 

 width of the chamber) and composed of a strip 

 of steel with its two edges bent downwards at 

 right angles : this is to make the steel rigid, and 



