64 



DISPLACEMENT INTERFEROMETRY BY 



Following the usual theory, if L has the form 47rw 2 / (1/nA) , n being the number 

 of turns of wire around an electromagnet of length /, area A, and permeability 

 /ji, and if the coefficient of mutual induction of the transformer is M= \/LiL t , 

 and if e = e sin wt is the impressed voltage, the secondary voltage will be 



cos (cof 



Hence the secondary current, when L 2 Rz and C 2 are there inserted, is 



e<> cos (at ip\ o^o) 



where 



= tan 



= tan 



This is further to be modified with reference to the amplitude and phase of 

 the extraneous coupling. 



46. Direct telephonic induction. The influence of the oscillating magnetic 

 field on the telephone is much more pervasive than one is apt to suppose. 

 The effect, moreover, is particularly marked if the telephone is open, i.e., with 

 no connection between the clamps. A coil of high resistance of telephone 

 wire, implying many turns, is naturally preferable. The stray vibrating field 

 produced by a small electromagnet (say 0.25 inch iron, 2 inches long) is quite 

 audible even beyond 50 cm. from the electromagnet. The degree of response 

 depends, moreover, on the orientation (fig. 70) of the telephone relative to the 



M M I* 



d' 



electromagnet E. If we take the three cardinal positions of the plane of the 

 coil or the diaphragm, the vertical positions e, d, f, and the fore-and-aft hori- 

 zontal positions h, g, i, have their maximum response in the plane of symmetry 

 gdE. The right-and-left horizontal positions d", b, a, c, d' give minimum 

 response (telephone silent) in this plane (Ed), with maxima at symmetrical 

 positions, 6 and c. Near E (20 cm.) the sounds may be quite loud. 



Although all telephones show the phenomenon pretty well, since it is more 

 distinct on open circuit (which implies a current oscillating from clamp to 

 clamp) it would be well worth while to wind a telephonic bobbin provided 

 with a capacity for the particular purpose of catching the stray magnetic 

 field, such as is here encountered. Without proceeding to this extent, I used 

 the telephone as a secondary, as shown in figure 71, where E is the electro- 

 magnet of the interruptor /, S being the vibrating break-circuit spring. The 



