EXPERIMENTAL STUDY OF HEAT LEAKAGE. 745 



possible to interchange the thermometers without disturbing the 

 set up. The oil, of which some 35 gallons are used, may be drained 

 into a lagged storage tank (not shown) by opening the cock A'l (Fig. 5). 

 After the thermometers are interchanged, the oil, still hot, is pumped 

 back into the bath-tank by means of an oil-pump (not shown). The 

 oil that has been used is a heavy tempering oil, quite viscous at room 

 temperature, and having a very high flash-point. 



In all of the work here reported, the control of temperature has been 

 by means of a mercury -in-glass thermostat of the familiar type, oper- 

 ating by means of a platinum contact, through two relays, to open 

 or close the circuit supplying the heating coils. Owing to the rather 

 large current (25-30 amperes) carried by this circuit, it was necessary 

 to use alternating current, to avoid troubles with arcing on the make 

 and break. The type of thermostat mentioned has been used with 

 entire satisfaction at temperatures up to 220° C, and, in some earlier 

 work, with fair satisfaction at 240° C. At higher temperatures, the 

 operation is likely to be uncertain because of evaporation of the 

 mercury. 



The secondary superheater (&, Fig. 5) in which the final adjust- 

 ment of temperature is made, consists of eight | in. copper pipes bent 

 back upon themselves as shown, and silver-soldered at both ends into 

 manifolds. The pipes are arranged in two groups, each group con- . 

 sisting of four pipes in parallel; these two groups may, by suitable 

 arrangements, be placed either in series or in parallel, thus making it 

 possible to double the velocity of steam flow. In the earlier form of 

 the apparatus, with the vertical or U-type, and also with the V-type, 

 of plug case, the secondary superheater consisted of a single coil of 

 f in. copper pipe, the total volume of which (0.052 ft.^) was not very 

 different from that of the copper pipes of So. The steam velocity 

 in this earlier superheater was necessarily much greater than the 

 velocity of flow in the talc -lined tube T, Fig. 1, or the tube T of Fig. 3, 

 where the diameter of the circular cross-section was about 1 in.; as a 

 result of this, the temperature of the steam at the high side ther- 

 mometer was higher than that of the oil, by an amount depending on the 

 tiow, because of the decrease in the kinetic energy of the steam as it 

 passed from the small cross-section of the superheater to the larger 

 cross-section of the tube T. It was chiefly in order to avoid this 

 defect, with its obvious possibilities, direct and indirect, that the 

 parallel arrangement of pipes was adopted in the later form of super- 

 heater. The velocitv of flow here is verv nearlv the same as in the 

 tube T of the plug case (Fig. 3), or as in the conical annular space 

 surrounding the plug, (Fig. 4), when the two groups of superheater 



