TELEPHONY 365 



ring is the core and the coil the primary. Hence powerful currents are induced in the material 

 placed in the trough, thereby heating it. Various forms of this induction furnace have been 

 designed by F. A. Kjellin and by the firm Hermann Rochling & Co. (called the Rochling- 

 Rodenhauser furnace), in which induced heating is combined with direct resistance heating 

 by passing an external .electric current through the charge. By means of the induction 

 furnace and by suitable admixtures of fluxes (lime) and oxidising agents (iron ore) crude 

 steel or scrap can be refined into a high grade of steel. For laboratory purposes forms of 

 resistance furnace have been devised by Heraeus, which consist of a porcelain tube wound 

 round with a platinum strip and enveloped over this with a non-conducting covering. When 

 a current is passed through the strip the tube is soon brought to a red heat. Crucible and 

 muffle furnaces are similarly made. Small arc furnaces in which an arc is formed between two 

 carbon electrodes included between two massive blocks of lime or dolomite are used in 

 chemical laboratories. The scientific work of H. Moissan was done with one of this type. 



Electric heating and metallurgical processes can be conducted in some cases by passing 

 a powerful electric current through the materials operated upon. The heat so produced 

 effects some desired chemical change. In other cases a crucible may be packed round with 

 high resistance material such as "kryptol," through which an electric current is passed and 

 heat thereby produced. The great advantage of such electric heating is that the heat can 

 be produced exactly where it is required and there are no products of combustion. 



In the case of electric arc furnaces there is a necessary loss of heat through the electrodes. 

 The rules for the most economical size have been discussed by C. Hering and A. E. Kennelly. 

 Electric arc furnaces are used extensively in France at La Praz and Ugines for producing 

 ferro-alloys, whilst induction furnaces are largely employed in steel manufacture. 



6. TELEPHONY 



In the last few years considerable attention has been paid to the theory of the prop- 

 agation of electric currents along wires and cables, especially with reference to the 

 rapidly alternating currents employed in telephony.. The necessity for this arose from 

 the costly nature of long telephone lines and cables, and the importance of being able 

 to predict the performance of such a line from its specification before construction. 



The mathematical theory has chiefly been developed by O. Heaviside, M. I. Pupin, 

 A. E. Kennelly, G. A. Campbell and others, and has been put into a form intelligible to 

 practical engineers by A. E. KennelLy and J. A. Fleming. The electric current which 

 flows in a telephone cable when transmitting speech is a very complex ebb and flow of 

 electricity. In accordance with Fourier's theorem this motion can be analysed into the 

 sum of a number of simple harmonic motions of various amplitudes and frequencies in 

 the ratio 1:2:3 et c. These are called the harmonics of the current. That which gives 

 the sound its meaning to the ear when transmitted telephonically is the wave form of 

 this variable current. This depends upon the relative amplitude and phase of the 

 harmonics. In an ordinary telephone wire or cable the harmonic oscillations of high 

 frequency travel faster than those of low frequency, especially if the cable has much 

 electrical capacity. Also the higher harmonics are more rapidly degraded or attenuated 

 in amplitudejthan the lower. Hence after travelling a certain distance the wave form 

 of the current becomes distorted so much that when it flows through the receiving 

 telephone it no longer reproduces the sound uttered to the transmitter. Heaviside showed 

 that this distorsion depended upon the relation of the resistance, capacity, inductance, 

 and leakance of the line, and that if a line were constructed in which the product of the 

 capacity and resistance per mile was equal to the product of inductance and leakance in 

 homologous units the line would be distorsionless. It is always found that for ordinary 

 lines the first named product is larger than the second. 



The practical problem of making a telephone cable more or less distorsionless was 

 found to depend upon the possibility of increasing considerably the quality called its 

 inductance, which is in other words the electric inertia of the line, or the quality in 

 virtue of which electric currents when created in it tend to persist and not die away 

 instantly. This can be done by " loading " the line or inserting in it coils of wire of 

 high inductance. It can also be achieved by winding over the telephone line with iron 

 wire, which process is called uniform loading. The first method was proposed and 

 worked out by M. I. Pupin, who showed that if the loading coils were placed at such 

 distances that 8 or 9 of them were covered by one wave length of the current in the line 



