450 Mr. R. S. Brough on the best Resistance 



that on short circuits the instruments should be wound with 

 thick wire, while on long circuits they should be wound with 

 fine wire. 



Professor Fleeming Jenkin, in his ' Electricity and Mag- 

 netism/ however, states that the resistance of the receiving- 

 instrument should not be more than a moderate fraction of the 

 resistance of the whole circuit. In a footnote, he adds that 

 some authority (unnamed) recommends that the resistance 

 of the receiving-instrument should be ^ of that of the whole 

 circuit, and remarks that this appears to be a very large value. 



Mr. Schwendler, in his ' Testing Instructions,' published 

 under the authority of the Director-General of Telegraphs in 

 India, taking into consideration the influence of want of perfect 

 insulation of the line-wire, deduces that the resistance of the 

 receiving-instrument should be f of that of the line-wire. 



The fact of the matter is, that on comparatively short lines 

 and at low speeds of signalling (say twelve words per minute) 

 the resistance of the receiving-instrument is not of much im- 

 portance, as deficiency of sensibility can be compensated by 

 increased battery-power, and the circuit will appear to work 

 equally satisfactorily whether the resistance of the receiving- 

 instrument be 500 or 2500 ohms. 



In such cases the general rule given in the text-books is 

 sufficient for practical purposes. 



When we come, however, to the case of high-speed signal- 

 ling*, or of very long and highly insulated lines, the question 

 assumes a different phase, and becomes one of great importance. 



Now the best resistance for an electromagnet to be employed 

 as a receiving-instrument on any line has to be considered 

 from two aspects, which may fitly be referred to as the "static " 

 and the " kinetic." 



Considered under the first aspect the problem is a purely 

 statical one: it is to find the resistance of the receiving-instru- 

 ment which will make its magnetic force a maximum when 

 a steady current is flowing from the sending to the receiving 

 station. By a steady current is meant one which does not 

 vary in strength with respect to time. This problem is com- 

 pletely solved and thoroughly understood. 



It can be shown! that the magnetic force is a maximum for 



L/Ii(l + 6)-"Vf+/a-e)-"V7J 



* For high-speed telegraphy, electromagnetic receivers are being su- 

 perseded by electro-chemical receivers, which are free from mechanical 

 and magnetical inertia. 



t Blavier, Annales TeUgraphiques, 1858, p. 234. 



