I. SPECIAL COLLECTIONS. 49 



213a. Ten Standard Plat Surface Gauges, from ^th of 

 an inch to T ^th of an inch varying xoV^th of an inch. 



They are standards for the thickness of sheet metal and the diameter of 

 small wire ; they serve for the correction of the wire gauge. 

 t Cylindrical standards are not made less than -j\jth of an inch in diameter. 



216. Standard Screw Gauge of Whit worth thread. 



This gauge is to show the form of the Whitworth standard thread. The 

 two cylindrical parts give the exact diameter of the top and the bottom of 

 the screw threads in universal use ; and the angle of the thread is 55, and 

 is rounded off th of its depth. 



211. Millionth Measuring Machine. 



The screw of this machine has 20 threads to the inch, the screw wheel 200 

 teeth, and the micrometer wheel is divided into 250, therefore each division 

 represents the one-millionth of an inch. 



TJie end of the fast headstock and the end of the movable headstock are 

 true planes parallel to each other ; the ends of the piece to be measured 

 must also be parallel true planes ; the feeling piece is a piece of steel about 

 T %ths thick, its sides being parallel true planes, and it is introduced between 

 the standard to be measured and the true plane at the end of the fast head- 

 stock ; when the proper adjustment has been made the movement of the 

 micrometer wheel one division, viz., one millionth of an inch, will cause the 

 feeling piece to be suspended friction overcoming gravity. 



The power of measurement and the true plane are the two great elements 

 in practical mechanics. 



An idea may be formed of the millionth of an inch, from the fact that 

 if a sheet of foreign letter paper were divided into 4,000 thicknesses, each 

 thickness would represent the millionth of an inch. 



APPARATUS USED BY DR. JOULE, F.R.S., FOR ASCERTAINING THE 

 MECHANICAL EQUIVALENT OF HEAT. 



218. Revolving Electro-magnet, used in 1843 for ascer- 

 taining the Mechanical equivalent of Heat. 



Part of the apparatus used in 1843 for the determination of the mechanical 

 equivalent of heat : viz., a revolving piece, holding a glass tube filled with 

 water, and containing an electro-magnet. This worked between the poles of 

 a powerful magnet ; and the heat evolved by the rotating electro-magnet was 

 measured by the rise of temperature of the water. In this manner the quan- 

 tity of heat lost by the circuit was ascertained when the machine worked as 

 an engine ; and, on the other hand, the quantity of heat produced when work 

 was done on the machine was also measured. 833 ft. Ibs. was the mechanical 

 equivalent of a degree Fahr. in 1 Ib. of water, as determined by these first 

 experiments. 



219. Calorimeter, containing a revolving agitator. This 

 was employed in the experiments on the heat evolved by the 

 friction of water, made in 1849. The equivalent arrived at was 

 772 ft. Ib. 



40075. D 



