Heat Vaporization of Water. 89 



are hinged to the bobbin by means of a double joint which 

 permits radial motion. A toggle joint, operated by a rod 

 inside the vertical shaft, connects the two rubbers diametrically 

 through an opening in the bobbin. This rod is forced upwards 

 by means of a strong spiral spring in the bottom of the bobbin, 

 and draws the rubbers in toward the center at the same time. 

 When pressed down by the controlling device at the top, it 

 forces the rubbers out radially. Surrounding the rubbers and 

 accurately turned to fit them, is a cylinder supporting a torsion 

 disk at the top. As the shaft rotates the rubbers move with it, 

 and on account of the friction drag the cylinder and the tor- 

 sion disk at the top with them. This motion is communicated 

 by the tapes to the pulleys of the bent levers and the pendu- 

 lums are displaced until their moment is equal to that of the 

 friction. The double hinged joints are the important feature 

 of this device. They permit the rubbers to seat themselves 

 perfectly in the cylinder and the resulting friction is very uni- 

 form. In fact, the small periods of its variations are so short 

 compared to that of the long bent levers that they are com- 

 pletely integrated by these levers, the record being almost a 

 straight line. The controlling device consists of a hand screw 

 to force down the rod operating the toggle joint. This pres- 

 sure is transmitted through the ball-bearing since the rod is 

 rotating with the shaft. 



The manipulation of the machine is quite simple. A steam 

 bath is allowed to flow through the chamber from the boilers 

 throughout the experiment, maintaining all parts inside the 

 shield at the temperature of the bath. The motor is started 

 and the load is gradually increased by the control to the desired 

 constant. When the water is evaporating freely and the ther- 

 mal conditions have been maintained constant for some time, 

 the weights are adjusted a little light and, as the water in the 

 cup evaporates and the pointer comes to the zero, the clutch 

 is operated throwing the counter and the recorder in gear, the 

 weights and the counter having been observed and recorded. 

 After any convenient period the weights are again adjusted a 

 little light and the clutch again operated just as the pointer 

 comes to zero, the counter and the weights being observed and 

 recorded. 



The calculation of the mass of the water evaporated is made 

 in the usual way, but that of the mechanical energy may need 

 a word of explanation. Since the ordinates and the abscissas 

 of this curve are linear functions of the friction and the num- 

 ber of the revolutions of the rubbers, the following relations 

 hold : 



W 



=f f .as 



