48 DISPLACEMENT INTERFEROMETRY BY 



32. Adjustable telephone. The question as to the most advantageous posi- 

 tion of the telephone is important. Consequently the telephone ih, figure 41, 

 was mounted on a stout flat steel spring controlled by a micrometer-screw. 

 By actuating this the face of the magnet could be approached as near g' as 

 permissible or withdrawn to a remoter position, with precision. The experi- 

 ments made at length showed that within a wide range of tensions and of 

 telephone positions a particular degree of approach of the telephone corre- 

 sponded to each particular stress of wire. Unless these paired positions are 

 selected, the bifilar system does not respond. Tense wires require a nearer 

 telephone; less tense wires a more remote telephone, within wide limits. 

 Therefore the condition of resonance may be reached either by adjusting the 

 telephone on the micrometer-screw for a given tension of wire or by changing 

 the tension of the wires for a fixed telephone. Curiously enough, there is no 

 marked difference of sensitiveness within the range in question. Naturally, 

 if the wires are too loose or the telephone too remote there will be no response. 

 It is not, therefore, possible to increase the sensitiveness by approaching the 

 telephone magnet to the armature as one would naturally suppose. If reson- 

 ance is reached the position of the telephone is rather a matter of indifference. 

 It is very important, however, that the tension of both wires is the same; other- 

 wise there is reduced sensitiveness. 



If the lens carriage gg', figure 41, is loaded appropriately, the sensitiveness 

 may be increased. The tension of the wires of the loaded system is increased 

 until resonance appears. On the other hand, such a system is apt to be annoy- 

 ingly subject to tremors, particularly during adjustment, and the maxima are 

 reached more slowly. A light carriage is thus in general preferable, unless 

 very low frequencies are to be matched. 



With a distance of not more than 50 cm. between slit and ocular, a judicious 

 disposition of parts eventually gave me 40 ocular scale-parts per virtual 

 milliampere, viz: 



so that here the above effect of frequency, n, is no longer apparent. 



Carrying the telephone from a remote distance (say 0.5 cm.) to the nearest 

 distance compatible with a free vibrating system (this eventually sticks to 

 the magnet), the deflection 5 in a given case increased from about s=i6 

 through 21 then falling to 7 for a very tense wire. The maximum is flat. 

 Tight wires require very accurate adjustment as to tension, but the full 

 deflection builds up very slowly in view of the remoter telephone. The system 

 is more subject to shaking during adjustment. 



33. Coil tester. An interesting application of the above apparatus, where 

 a definite frequency is usually assigned, is its possible use for measuring the 

 magnetic fields of different coils. For this purpose I wound a long, thin solen- 

 oid or primary of an induction coil, which when inserted into the coil to be 



