52 A. G. Bell— Exploring for an imbedded bullet. 
The difficulty of adjusting the coils led me ultimately to the 
idea of the apparatus shown in figs. 21, 22, 23, 24, which is 
the most practical form of the instrument yet devised. 
The two exploring coils A B (fig. 21) are arranged as shown, 
in a recess turned out in a single block of wood C. 
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The coils are temporarily connected with a telephone bat- 
tery and rheotome in the manner shown in fig. 1, so that they 
may be adjusted by hand to form a balance. When they 
have been arranged in their position of silence, the hollow 
in the block of wood C (fig. 21) is filled with melted paraffine. 
Upon cooling, the two coils are found immovably fixed in one 
solid cake of paraffine. 
As a matter of practice it is found impossible to fix the coils 
in this way exactly in their position of silence; but by means 
of two other very small coils, D E (fig. 22), of insignificant 
resistance, forming a sort of fine adjustment external to the 
explorer, a perfect balance is easily obtained. In this instru- 
ment the swaying of the coils A B produces no effect upon the 
balance. 
The completed arrangement is shown in plan in fig. 22, and 
the explorer and balancing coils are shown separately in per- 
spective in figs. 23 and 24. 
On account of the small size and slight resistance of the bal- 
ancing coils we were enabled to make the adjustable parts of 
the balancer of metal without practical interference with the 
sensitiveness of the exploring instrument, and this gave us the 
