35 8 ELECTRICAL ENGINEERING 



superheater most frequently fitted in marine boilers is, however, thai known as the 

 " Schmidt " superheater. This is a modification of the Schmidt superheater so general- 

 ly employed on the Continent of Europe, for locomotive boilers, which is also now being 

 introduced for the same purpose into England. 



In the marine type, the pairs of headers are placed in the smoke boxes of the boilers. 

 The superheating tubes are of small diameter and are made of seamless steel. They are 

 formed in a series of hair-pin loops, each loop being placed in one of the smoke tubes of the 

 boiler, extending to within about nine inches of the fire box end. The arrangement is such 

 that each of the series consists of 4, 5 or 6 loops according to the design of the boiler, the 

 total number employed being sufficient to provide each of the smoke tubes with its superheat- 

 ing tube. In a two furnace boiler only one pair of headers is used, placed in front of the 

 space between the nests of tubes, and the superheating tubes are symmetrically placed on 

 both sides of the headers. In three furnace boilers two pairs of headers are used, and the 

 superheater tubes are more closely placed on one side of the header than on the other to suit 

 the arrangement of the smoke tubes. 



For use in smoke tubes 2 inches outside diameter the superheating tubes are made 18 mm. 

 outside diameter and 2 3 mm. thick. Where 3j inch smoke tubes are used the superheater 

 tubes are 23 mm. outside diameter and are 3 mm. thick. The total superheating surface 

 is usually about two fifths of the heating surface of the boiler proper. 



The employment of this form of superheater makes very little difference in the design 

 of the boiler, with the exception that the ratio of heating surface to grate surface is somewhat 

 less than where no superheating is employed. 



In locomotive boilers the design has to be considerably modified. Many of the usual 

 upper small tubes are replaced by three rows of large tubes which are generally from 4^ 

 inches to 5! inches in diameter for the greater portion of their length, reduced about \ inch 

 in diameter for about 10 inches at the firebox end to permit of better water circulation over 

 the tube plate. In each of these large tubes one section of the superheater is placed. This 

 consists of a long seamless steel tube shaped as a four-fold bend. The two ends of the tube 

 are bent upwards and are expanded into one rectangular flange which by means of one central 

 stud and nut is secured to the header. In this case one header only is used, placed in the 

 smoke box. It is made of cast iron of the special quality used for cylinders, and is formed 

 with two passages, one serving as the inlet for the saturated steam and the other as the outlet 

 for the superheated steam. 



The practice with locomotive boilers is to make the superheating tubes with about one 

 fifth of the total heating surface of the boiler. The damper retards or entirely cuts off the 

 flow of furnace gases past the superheating tubes when there is little or no demand for steam 

 from the boiler. Whenever the Schmidt superheater is used arrangements are made for 

 keeping the superheating tubes and the enclosing smoke tubes free from soot and scoriae 

 by means of a steam jet from a flexible tube. (J. T. MILTON.) 



ELECTRICAL ENGINEERING 1 



i. ELECTRICAL MEASUREMENTS 



The importance of exact measurement in all electrical work has led to great attention 

 being paid to improvements in methods for measuring electric currents, voltages, and 

 power, especially in connection with alternating currents of low and high frequency. 

 The direct reading potentiometer has been of such use in all direct current measurement 

 that the invention of a similar instrument for alternate current work has long been de- 

 sired. This has now been provided in Dr. Drysdale's alternate potentiometer. 



This instrument consists of an ordinary direct current potentiometer, but has an elec- 

 trodynamometer in series with the slide wire to adjust an alternating current to the same 

 root-mean-square value as the direct current, which makes a known fall of potential down a 

 certain length of the wire. In addition to the usual resistances for regulating this current, 

 the alternating current is supplied through a phase-shifting transformer, which enables its 

 phase to be varied through 180. If then it is desired to measure an alternating current 

 voltage, which may be reduced in any required ratio by a transformer, this voltage is bal- 

 anced against the fall in voltage down the slide wire which is produced when the slide wire 

 is traversed by an alternating current taken from the same source so as to have the same 

 frequency. To do this the phase as well as the strength of the alternating current in the 

 slide wire must be capable of adjustment. The balance is determined by using a vibration 

 galvanometer in place of the movable coil galvanometer employed with direct currents. 

 The absolute value of the fall in potential down known lengths of the slide wire is obtained 

 in the usual way by a direct current and the aid of a standard Weston cell. Hence if we 



1 See E. B. articles referred to under PHYSICS: Electricity in Index Volume, p. 940. 



