60 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



(e, e). The device by which any movement of (c) in a horizontal direc- 

 tion is prevented is seen in the upper part of the figure. The two nuts — 

 (/) and (g) — are threaded internally to fit c. 

 The lower and wider one is bored at two oppo- 

 site points for two rods, of which only one (h) is 

 seen in the figure. Corresponding to the holes 

 in g, two other holes are bored in the brass cap (i) . 

 Having found the correct position for the con- 

 tact-point, the rods h and hi (not seen), are in- 

 serted, and the set nut (/) is turned down upon 

 g. The rod (c), though still free to move in a 

 vertical direction, can not now turn with the 

 adjusting nut (e, e). 



Sparking at the point of contact in the ther- 

 mostat is effectively prevented by spanning the 

 spark gap (Figure 29) with a lamp of high volt- 

 age (h) and a condenser (ci). Since removal 

 of the condenser has not been found to induce 

 visible sparking at the point of contact, it is 

 doubted whether it serves any useful purpose. 

 As a matter of fact, it is often omitted. The 

 lamp (li) which is ordinarily employed is one 

 of 16 candle-power at 250 volts. 



The water in the baths is always in rapid 

 motion, and a thermostat which is immersed 

 in it without protection is subject to slight but 

 constant jarring, which results in a phenom- 

 enon which has come to be known under the 

 name of "frosting." The air between the mer- 

 cury in the thermostat and the glass wall col- 

 lects at a multitude of points in minute bubbles, 

 which give to the glass a frosted appearance. 

 In the course of time, the bubbles of gas coalesce, 

 forming aggregations so large that the true 

 nature of the phenomenon can be discovered by 

 the naked eye. The first indication which one 

 usually receives that ' 'frosting " has commenced 

 is a "chattering" of the relays. 



11 Frosting" can be prevented by protecting 

 the thermostats from the shock of the moving 

 water by surrounding them with metallic tubes, 

 or by exhausting them before introducing the 

 mercury. The boiling-out process employed 

 for barometers has not been found practicable for thermostats of the 

 form used by us. 



Fig. 30. — The thermostat. 



(a) Platinum rod, pointed at 

 lower end ; (b) glass guide to 

 keep (a) in center of tube; 

 (c) threaded brass rod ; (e) in- 

 closed nut for adjusting con- 

 tact point; (/), (g), and (h) 

 arrangements for preventing 

 all movements of the contact 

 point, except in a vertical di- 

 rection. The figure to the 

 left shows the glass parts of 

 the thermostat. 



