l82 



SCIENTIFIC NKWS. 



[Oct. 1st, i£ 



gations, published in No. 65 of the Journal of f/re Society of 

 Telegraph Engineers, should be consulted. The succeeding 

 discussion will be found more amusing than is commonly 

 the case. 



Although the use of a variable resistance transmitter is 

 a great improvement over the Bell transmitter on short 

 lines, very little benefit results from its employment on lines 

 more than a mile or two long, and beyond a certain length 

 the Bell performs at least as well. The reason is that the 

 variation of the current is proportional to the amount of 

 variation in the total resistance of the circuit, including the 

 battery, the transmitter itself, the line, and the receiving in- 

 strument. The resistance of most transmitters lies between 

 2 and 30 ohms or units, and the total variation due to the 

 air vibrations is not more than 25 per cent, of this. As the 

 resistance of a mile of the wire usually employed for tele- 

 phonic purposes is from 15 to 20 ohms, the resistance of the 

 receiver from 50 to 70 ohms, and the battery 6 to 10 ohms, 

 it will appear that on a line only a few miles long the ripples 

 set up in the constantly flowing current by the variations of 

 the transmitter resistance are very small. Little is gained 

 by using a more powerful battery, as the increased mini- 

 mum current holds the receiving diaphragm with greater 

 force, and so renders it less sensitive. 



The use of an induction-coil was very soon suggested as 

 a means of increasing the transmitter's utility, and it is now 

 all but universally used. Who invented this application is 

 not quite clear. Possibly the idea occurred to more than 

 one man almost simultaneously. At any rate, the inventor 

 deserves the thanks of his contemporaries, for he made the 

 world a present of the invention, as Professor Hughes did 

 of his invention of the microphone. 



In describing the Bell telephone, it was stated that a 

 change in the magnetic conditions surrounding a wire might 

 produce a current in that wire, and also that a wire carrying 

 a current produced a magnetic effect in its neighbourhood, 

 magnetising iron, deflecting a magnetic needle, or attracting 

 iron filings. One result of these two effects is that, if two 

 wires run parallel to each other, a change in the strength of 

 a current carried by one produces a tendency to a current 

 in the other ; if the second wire forms part of a closed cir- 

 cuit, a current actually flows. This current is called a 

 secondary, or induced current. It continues as long as the 

 first, or primary, current is changing in strength. Its 

 strength is proportional to the rapidity of change of the 

 primary (we might put this in the language of the differen- 

 tial calculus, but it would not be any truer) to the length ol 

 the parallel parts of the two wires, and to the relative dis- 

 tance form, and environment of the wires. The nearer to- 

 gether they are, or the more easily magnetised the surround- 

 ing bodies are, the greater will be the inductive effect of the 

 primary circuit. So long, however, as the primary current 

 is steady, no secondary current will be produced. It is 

 found that the best form for telephonic induction-coils is a 

 simple bobbin of some insulating material, such as wood or 

 ebonite, with a core of iron wires running through its centre, 

 and with the two wires wound on it in succession. The 

 primary wire is usually wound on first, and is a compara- 

 tively short, thick wire ; the secondary wire is wound over 

 the primary, and is very much finer and longer. The pri- 

 mary is thus oi low resistance, and a great length of secon- 

 dary wire is arranged parallel to a small length of primary 

 wire. The effect of the iron core is to increase the magnetic 

 eflect of the primary current variations, and so to increase 

 the inducting power. That it has this effect proves that the 

 inductive action is not directly electrical, but that a magnetic 

 effect intervenes. A little induction-coil can be easily made 

 by taking an ordinary empty cotton-reel, filling the central 

 hole with a bundle of fine iron-wire, and winding into the 



space formerly occupied by the thread first a length of say 

 No. 20 B.W.G. silk-covered copper wire, and then a length 

 of No. 36 or 40 silk-covered wire. This will be better if a 

 boxwood or ebonite bobbin is employed, with the central 

 tube very thin and the copper wire well served with paraffin 

 wax. If, then, an ordinary galvanic-battery cell is put in 

 circuit with the thick wire, a galvanometer connected to the 

 ends of the fine wire will show a deflection each time the 

 primary circuit is made or broken. A small shock can be 

 felt at those moments, and other proofs of the secondary 

 current obtained. 



Now if the primary wire of such a coil is put in circuit with 

 a battery and a variable resistance transmitter, the transmitter 

 will probably be by far the highest resistance of the circuit, 

 and its resistance variations will cause a fairly large per- 

 centage variation in the current. As increases and decreases 

 in the primary current cause currents of opposite direction 

 in the secondary circuit, the result of the current variations 

 in the primary circuit will be to cause alternating currents 

 to flow in the secondary circuit, corresponding, of course, 

 to the motions of the transmitter's diaphragm, and capable 

 of imparting similar motions to the diaphragm of the receiv- 

 ing instrument. The advantages obtained are that by 

 suitable winding of the induction coil and increase of the 

 battery power, the induced currents can be made compara- 

 tively powerful, and, as there is no permanent current flow- 

 ing through the receiving instrument, its sensitiveness is 

 not interfered with by the attraction of the magnet for the 

 armature. This combination of variable resistance trans- 

 mitter, battery, induction coil, and Bell or similar receiver, 

 is, therefore, the best yet made, and is indeed the only one 

 of practical use for any lines of respectable length. 



In the next article, some of the most useful practical 

 arrangements, including the exchange system, will be de- 

 scribed. We may conclude by putting down in tabular form 

 again the cycle of action in a telephone circuit with a vari- 

 able resistance transmitter, and a battery as the sending 

 arrangement, and also with an induction coil added : — 



Mechanical Operations : — 



Air vibrations ) ^on^grted into vibrations of transmitter diapliracm— 



(or changes of pressure) ) ^ ^ 



And variations of pressure on transmitter contacts — 

 Causing — 

 Variations of electrical— 

 Electrical :- 



Resistance of transmitter contacts, and consequent variations of current- 

 operations — 

 Strengtii— resulting in— variable magnetisation of receiver magnet and— 

 Mechanical:— 



Vibrations of receiver diaphragm — converted into air vibrations. 



The introduction of the induction coil adds the following 

 steps, between the current variation and the magnetic 

 effect in the receiver :— 



Variations of current strength in primarj- circuit— inducing— 

 Alternating currents in secondary circuit— resulting in- 

 variable magnetisation of receiver magnet. 



It will be noticed that there is no direct transfer of energy 

 from the transmitting to the receiving diaphragm ; the 

 battery gives out the energy utilised in the receiver ; the 

 transmitter merely controls the amount given out. There 

 is thus no direct relation between the two amounts of 

 energy, and more work may be done in the secondary 

 circuit than is given by the vibrating air to the transmitting 

 diaphragm. Hence it is that an actual magnifying of sound 

 may be obtained by suitable arrangements, and that a 

 musical performance, at a distance of many miles, may be 

 rendered audible to a roomful of people. 



(To be conti'/iucd) 



