274 Scientific Procecdings, Royal. Dublin Society. 
either by placing it in the line of dip or by an adjacent magnet 
the existence of the crack gave rise to a secondary magnetic pole, 
which was at once revealed by the telephone. The magnetic 
method of discovering flaws in iron was some time ago suggested 
by Mr. Saxby, and the foregoing experiment shows that the tele- 
phone may, with considerable advantage, replace the magnetic 
needle. It is obvious that to engineers this is a point of much 
importance, and, I hope, the experiment will be tried on some 
large iron forging or casting. During the magnetization of the 
bar it is interesting to notice the kind of breathing sound which 
is heard in the telephone, suggesting that possibly the telephone 
may be of some use in investigating the molecular changes that 
accompany the act of magnetization. 
Fig. 2. 
Measurement of resistance—By using the telephone and in- 
terruptor instead of the galvanometer in a Wheatstone’s bridge, 
the resistance of wires may be measured with great ease and 
accuracy. The sensitiveness of the telephone is shown in the 
following arrangement, Fig 2:—The interruptor J and telephone 
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