Meters and other Electrical Instruments, 9 



and the height 9 centim. The B given in the table is the 

 average for the whole cross-section of the coil at the middle 

 of its height. The total flux corresponding to this was 17,500. 

 B at the centre of this cross-section was 53. 



In addition to the instruments already discussed, experi- 

 ments were also made upon a number of meters of different 

 types, some being for direct, and some for alternating 

 currents. As some of the types vary in construction from 

 year to year, a few words of description in each case will make 

 the results clearer. 



Aron Watt-hour Meter (1894 type). — Range to 50 amps. 

 at 100 volts. Two pendulums, each carrying shunt-coils, 

 are acted on by series-coils under them, one being accelerated 

 and the other retarded. With both series and shunt-currents 

 passing (at full load) the mean B between the fixed and 

 movable coils was about 70. 



Frager Watt-hour Meter. — Range to 10 amps, at 100 volts. 

 A meter of the " Feeler " type, in which the shunt and series 

 coils form an ordinary wattmeter, whose deflexions are 

 integrated at intervals, The mean B was measured as near 

 the centre of the shunt-coil as possible. 



At full load B = 63, 



With shunt-current alone . B = 13, 



With series-current alone . . B = 50. 



Hookham Direct- Current Ampere-hour Meter. — Bange to 

 100 amps. A small disk, surrounded by mercury which 

 carries the current, is cut twice by part of the magnetic flux 

 from a strong permanent magnet (of cross-section 7*5 sq. 

 centim.). The disk is thus caused to turn. On the same 

 spindle is a brake disk of copper (5*4 centim. diameter), 

 which is also cut by a part of the flux from the same perma- 

 nent magnet. Unfortunately it was not possible to take the 

 meter to pieces, so the driving flux could not be measured. 

 By slipping a search-coil along the permanent magnet, the 

 total leakage was found to be over 26,000 lines. The total 

 brake flux passes through the disk at four air-gaps, two and 

 two in series, and has a value of about 5000 lines. The 

 8 pole-pieces which direct the flux have cross-sections of 

 1*53 centim. each. In two of them the iron near the air-gap 

 is turned down so as to leave only a thin neck of about 0*12 

 sq. centim. cross-section. This is supposed to increase the 

 permanence of the flux. At these necks more than J of 

 the total flux leaks from the iron. Whether they increase or 

 diminish the permanence seems quite uncertain. The mean 

 flux density at the four air-gaps was found to be about 1020. 



