Bickerton. — On a new Thermometer* 153 



but there are many cases to which it does not apply well, nor is it well seen 

 in flat rooms, in which most elementary teaching is carried on ; it is also 

 expensive, and requires some skill to use satisfactorily. These considerations, 

 and the necessity I have often felt for such an instrument, induced me to 

 design the thermometer described below. The thermometer depends upon the 

 different coefficients of expansion of metals, and is constructed as follows : 



About a dozen pairs of thin strips of zinc and steel, about 4 inches long, 

 are riveted at their alternate ends, the two outside strips are continued on 

 beyond one end of the bundle so formed. Each of these two pieces have a 



steel pin soldered across its end ; these two pins work in holes in the end of a 

 long index finger, which at the mean temperature of the range of the 

 thermometer is in a straight line with the bundle of metal strips. Any 

 increase of temperature causes each of the zinc strips to expand more 

 than the steel, and the sum of their difference is at once transmitted to the 

 pins, one of which is thus thrust beyond the other, and so causes the index 

 to move on one side. When the temperature is lowered the index moves to 

 the opposite side. The bundle of strips is filed up into a cylinder and placed 

 in a copper tube, the bottom of which is closed. At the top of the tube is 

 fixed a fan-shaped piece of metal, on which is marked the scale of temperature. 

 The index moves backwards and forwards in front of this scale. The 

 accompanying sketch shows the construction of the thermometer. 



When the thermometer is used before a large 

 audience the size of the scale may be increased by 

 fixing to the back of the permanent scale a quadrant 

 of black paper having white figures on its circum- 



ference. 



!>e leng 



by a strip of white paper pasted on it. With these 

 additions the movement of the index produced by 

 an alteration of a few degrees of heat may easily 

 be seen by an audience of any number. I have 

 used the thermometer to illustrate the following facts : — 1st. It offers a good 

 means of showing, by the principle of its construction, the difference of 

 expansion of different metals. 2nd. The constant temperature of fusion and 

 boiling. Thus the thermometer is placed in ice at —10° C, heat is applied, 



the index moves up to zero, it then remains still till all the ice is melted, it 

 then goes on moving up to 100° C. 3rd. A pair shows well the different rate 

 of cooling of water in polished and blackened tin tubes. 4th. The cold 

 produced by the fusion of solids on ordinary freezing mixtures. 5th. The 



diff( 



thermometer 



be 



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