128 PROCEEDINGS OF THE AMERICAN ACADEMY 



Tables I. and II. give the results of some experiments made on 

 cables ; Table III., on pole lines ; and Table IV., on mixed cables and 

 pole lines. The numbers showing the total resistance and total ca- 

 pacity refer to the line between the terminal instruments, and do not 

 include the terminal instruments. 



I find from these experiments, that the readiness with which tele- 

 phonic conversation may be carried on over any circuit, whether made 

 up of cables or pole lines, or both, depends, — 



1. On the total electrical resistance of the circuit joining together 

 the two stations. 



2. On the total electrostatic capacity of this circuit. 



So long as the insulation is sufficiently good to prevent any consid- 

 erable loss of current, its actual value, whether high or low, does not 

 affect the readiness with which conversation may be carried on. High 

 insulation is desirable, however, when two or more wires run near 

 together, in order to prevent extraneous currents from leaking in and 

 causing disturbing noises. 



The distance over which telephonic conversation may be carried on, 

 being thus dependent on tlie resistance of the circuit and on the ca- 

 pacity of the circuit, (being inversely proportional to each of these,) is 

 dependent upon their product, and this product has a definite numeri- 

 cal value for each kind of transmitter used, being 4,500 for trans- 

 mitters of the Hunnings type, and 2,000 for transmitters of the Blake 

 type ; that is, the product of the total resistance of the current be- 

 tween two telephones, by its total capacity, must not exceed 4,500 if 

 transmitters of the Hunnings type be used, and must not exceed 

 2,000 if transmitters of the Blake type be used. These results sup- 

 pose the ordinary Bell hand telephone to be used as a receiver, but 

 are not essentially varied by the use of other good forms of magneto 

 receiver. These limits are given for good business conversation. It 

 is of course possible for experts to get messages through circuits the 

 product of whose resistance and capacity is somewhat greater than 

 this. 



The resistance and capacity measured should be, as stated, that of 

 the line between the two telephones. It includes, of course, the re- 

 sistance and capacity of any way station switchboards or call bells. It 

 does not include the resistances of the terminal telephone instruments 

 used. 



These results may be briefly formulated in the following rule : — 



"No matter what may be the distance between two points, good 

 business conversation may be carried on between them, provided they 



