154 



Mr. W. H. Preece. On the 



[Mar. 3, 



where d is the diameter of the wire and h the height above the 

 ground. Taking h at 500 cm. and d\ at 0"243 cm., the capacity comes 

 out for 12^ wire at 0*0113 instead of 0*0124 microfarad per mile. The 

 cause of this difference is referred to further on. 



It then became necessary to determine the speed of the current 

 through wires of different lengths, resistances, and capacities. 



A multiplex distributor — such as we are now using in the Post 

 Office — enables this to be done with great accuracy. At each station 

 a circle is broken up into 162 sections, and an arm in connexion with 

 the line wire carrying a brush sweeps over these sections and makes 

 contact. This arm can be made to rotate at any speed. If it makes 

 three revolutions per second it will make the duration of each contact 

 ¥ |-g-th of a second. Now the two distributors are kept running in 

 absolute synchronism, so that the brush at each station always rests 

 simultaneously on corresponding sections. The sections at one station 

 can be placed in contact with the battery, and those at the other 

 station with a galvanometer or other sensitive apparatus. If the cur- 

 rent traversed the wire instantaneously then it would appear on the 

 same section at the same time on the galvanometer ; but the current 

 is always retarded, and the amount of retardation or the value of a can 

 be measured and the curve of arrival and cessation drawn by watching 

 the indications of the galvanometer on each succeeding section. 



The following table summarises a large number of experiments 

 that have been made in different parts of the country, and on different 

 lines, to determine by observation the connexion that exists between 

 speed of current, distance spoken through, resistance, and capacity. 

 The maximum current, or the crest of the wave, was observed. This 

 was equivalent to 0'003 ampere. 



It will be seen that the limiting distance through which it is 

 possible to speak varies inversely with the speed of the current, and 

 that the speed of the current varies inversely with the product of the 

 total resistance and the total capacity of the circuit. Hence we 

 can say that the number of reversals that it is possible to send 

 through any circuit varies inversely with the product of the total 

 resistance (R) and the total capacity (K). or the limiting distance 



S == KR x constant (1) 



This is only another form of Thomson's law for K = Ik, and 

 R = lr, and 



.*. S = JcrP x constant. 



It is seen that when the speed of the working current was 



0*001" speaking was perfect, 

 0*002" speaking was good, 

 0*003" speaking was fair, 

 0*004" speaking was difficult. 



