1883.] Theory of Magnetism based on New Researches. 191 



In fig. 3, two flat primary coils A', A" are near two similar 

 secondary coils B', B", the distance between these coils being 

 regulated by means of adjusting screws. The coils of B', B" are 

 wound in contrary direction to each other, consequently the induced 

 currents in each coil, if of equal strength, neutralise each other ; as- 

 shown by the arrow, the induced currents act upon the telephone C. 

 The primary coil A' is joined through the battery D to the rheotome 

 E and coil A". 



If we introduce a wire or bar of iron F' in the coils A', B', the in- 

 duced currents are increased by the magnetic conductivity from 

 the upper to the lower coil, and as the coils A', B' and A'', B" can 

 be at any desired distance, or from 1 millimetre to 1 metre apart, 

 we can test the conductivity of iron through any desired length. 

 If we introduce into the second pair of coils A", B /f a piece of iron 

 of the same form, size, and molecular structure as that already 

 in A', B', its effect on induced currents equals those of the first pair 

 of coils and we have a perfect zero, but in practice we find that there 

 is always some slight difference, even when the pieces are cut off the 

 same bar; we have to compensate for this difference, and thus- 

 measure the differential structure. 



In fig. 4 we have three coils, being the form adopted in my sono- 

 meter ; the secondary coil B being equidistant from the primary coils 

 A', A", acts upon the secondary coil in reversed direction, consequently 

 a perfect zero of effect is found whenever the action of A' equals that 

 of A", and this can be easily found by displacing either coil. If we 

 introduce a bar of iron between the coils A' and B, the balance no 

 longer exists, owing to the greater electromagnetic conductivity on 

 that side, but if the bar of iron F' F" is passed through the centre or 

 axis of all the coils, as shown in the diagram, we have no effect 

 except that due to a differential conducting power on one of its sides, 

 the direction and force of which can be found by the amount of dis- 



