of Duplex Telegraphy. 4 3 



It will now be clear that the given solution fulfils the follow- 

 ing essential conditions : — 



(i) Any variation of the resistance in the total system has the 

 least possible disturbing effect on the receiving-instrument . 



(ii) Any disturbance of balance can be eliminated by an appro- 

 priate movement of the two acting coils or their armatures, without 

 disturbing balance in the distant station. 



(iii) Conditional maximum magnetic moment of the receiving- 

 instrument. 



(iv) Conditional maximum current. 



Addendum. 

 Here I wish to give some additional explanations and correc- 

 tions with reference to the first and second parts of this investi- 

 gation. 



Journal of the Asiatic Society of Bengal, vol. xliii. part 2, 1874. 

 Page 12. I have substituted 



c' = L' + p" 

 without stating that this expression for c' is only approximately 

 true. The correct expression for c' is clearly 



C - l + i + l" + p» ' 



which approximates closely towards 1/ + p" if /" + p" is suffici- 

 ently small as compared with i. This for any line in good elec- 

 trical condition will always be the case. 



Page 9. In the footnote (Phil. Mag. August 1874, p. 125, 

 line 7 of note), for "as nearly as possible equal" read "as 

 nearly as possible proportional" 



Page 20 (Ibid. p. 137). 



d ^=Ua*-g*)+2a<!(d-g)=0, 



should be 



Y =L(« 2 -/) + 2a(^-/)=0. 



Pages 19 and 224 (Phil. Mag. August 1874, p. 125, and Feb. 

 1875, p. 115), after having shown that 



a+f=9 + d, 



I conclude at once that on account of equation (VI) {ad—gf=0), 



a=g = d=f (VIII.) 



while mathematically it follows only that 

 a=g and d=f 

 * Phil. Mag. February 1875, p. 113. 



