The Telephone as an Instrwment of Precision. 273 
with the telephone, and by means of a friction break, B B, the 
interruptions can, at pleasure, be reduced in rapidity. 
Determination of the “meridian” and “ dip.”—A coil of wire, 
capable of rotation on an axis passing through the centre of the 
coil, was joined in circuit with the telephone ; on rotating the 
coil, without the interruptor, a sound was, of course, heard in the 
telephone at each semi-revolution, from ‘the alternating currents 
set up by the motion of the coil through the terrestrial magnetic 
field. Inserting the interruptor, the clicks in the telephone were 
made continuous and much more audible. Placing the axis of the 
coil vertical, and turning the supports of the coil in azimuth, the 
positions of maximum and minimum action were easily ascer- 
tained, corresponding to directions, the former at right angles to, 
and the latter in the plane of, the magnetic meridian. By care- 
ful listening, the magnetic meridian could be found with a con- 
siderable degree of accuracy. In like manner, by inclining the 
axis of the rotating coil, the plane of the dip could be found from 
the marked alteration in the loudness of the sound. By using 
the points of minima a fair approximation (within a degree or 
two) to the angle of the dip was obtained. Obviously, the 
minimum number of lines of force were cut when the axis of the 
coil coincided with the plane of the dip, and the straight line 
bisecting the two supports of the coil coincided with the magnetic 
meridian. Although ‘this experiment could have been as well 
performed with a reflecting galvanometer, it is a good class 
illustration of the use of the telephone, which, moreover, is a far 
cheaper and more easily used instrument than a delicate galvano- 
meter. 
It is well known that to obtain the maximum energy from a 
magneto-electric machine, the commutator must be properly 
adjusted ; the telephone inserted in a derived circuit from such a 
machine, could enable workmen to adjust the commutator 
accurately and speedily; furthermore, a ready experimental 
method is furnished of testing any projected improvements in the 
working parts of a magneto-electric machine. 
Detection of flaws im vron.—Encircling a bar of iron with a coil 
of wire, with the telephone and interruptor in circuit, the exist- 
ence of magnetic poles in the bar could be detected by moving the 
eoil to and fro. When the bar was cracked, and magnetized, 
