362 PEI.I. SYSTEM TECIIMCAI. JOVRXAL 



compared with Curve a which is tlic arrival curve of a non-loaded 

 cable. The straight vertical .part of C'ur\e h represents the "head" 

 of the signal wa\e which has travelled over the cable at a definite 

 speed and with diminishing amplitude. The definite head of the arrival 

 curve is the most striking characteristic difference between the ideal 

 loaded and the non-loaded cable. In the latter, as is evident from 

 Fig. 3, the current at the receiving end starts to rise slowly almost 

 as soon as the key is closed at the transmitting end. When an e.m.f. 

 is applied to the sending end of the non-loaded cable a charge spreads 

 out rapidly over the whole length, the receiving end charging up 

 much more slowly than the sending end on account of the resistance 

 of the intervening conductor. Hence, if a signal train consisting of 

 rapidly alternating positive and negative impulses is applied to the 

 sending end, the effect at the receiving end of charging the cable 

 positively is wiped out by the succeeding negati\e charge before there 

 has been time to build up a considerable positive potential and the 

 successive alternating impulses thus tend to annul each other. In 

 the loaded cable the effect of inductance is to oppose the setting up 

 of a current and to maintain it once it has been established, and 

 thus to maintain a definite wave front as the signal impulse travels 

 over the cable. Hence, with inductive loafling the strength and 

 individuality of the signal impulses are retained and a much higher 

 speed of signalling is possible. It should be noted that by speed of 

 signalling is meant the rapidity with which successive impulses are 

 sent and not the rate at which they travel over the cable. This speed 

 of travel is actually decreased by the addition of inductance, about one- 

 third of a second being required for an iin|)ulso to traverse the \ew 

 York-Azores cable from end to end. 



It should be noted that Curve b of F"ig. ?> is for an ideal loaded 

 cable in which the factors of resistance, capacity and inductance are 

 constant. In a real loaded cable none of these factors are constant 

 and the arrival curve cannot be simply and accurately computed. 

 Even the capacity which is usually assumed as constant for real 

 cables varies appreciably with freciuencies in the telegraph range, 

 and owing to the fact that gutta pcrcha is not a perfect dielectric 

 material its conductance, which is also variable with frequency, must 

 be taken into account. Although the inductance of the cable is sub- 

 stantially constant for small currents of low frequency, it is greater 

 for the high currents at the sending end of the cable on account of 

 the increase of magnetic permeability of the loading material with 

 field strength and is less at high fre(|uencies than at low on account 

 of the shielding effect due to eddy currents. The resistance is highly 



