Mr. O. M. Mitchel 07i the Velocity of the Electrical Wave. 285 



plate. This receiving magnet was operated on in two modes, 

 at the pleasure of the experimenter, as follows: — 



1. By a local circuit, which was closed by the metallic 

 handle of the standard pen. 



2. By the long circuit before described, passing to and from 

 Pittsburg, a distance of 607 miles, along the wires. 



By these connexions, it will be seen that the clock-beats 

 were directly recorded by the standard pen. They were also 

 recorded by the variable pen (as 1 shall designate the second 

 one), moved by the standard pen, closing either a short local 

 circuit through the receiving magnet, or the long Pittsburg 

 circuit, through the same I'eceiving magnet, — this receiving 

 magnet, as before stated, closing the local circuit operating 

 on the variable pen. 



The standard pen record was followed by the variable pen 

 record, at an interval in time equal to the armature time of 

 the standard pen, increased by the armature time of the re- 

 ceiving magnet, increased by the wave time of the fluid in 

 passing through the short circuit and receiving magnet, this 

 last being of course insensible. This statement applies when 

 the variable pen is driven by the short local circuit. 



When the long circuit operates on the receiving magnet, 

 and through this on the variable pen, then the standard pen 

 is followed by the variable pen at an interval identical with 

 the preceding, increased by the time required by the electrical 

 wave or current for traversing the wires 607 miles. 



This statement is only true on the following conditions, 

 viz. — 



1. The intensity of the local circuit and the long circuit 

 must be reduced to equality. 



2. The adjustment of the receiving magnet must be con- 

 stant, and its pass must be reduced to a minimum. 



These two conditions being fulfilled, in case the two pens 

 are so adjusted to each other in position that a straight line 

 joining any two corresponding dots struck by them on a disc 

 at rest will pass through the centre of the disc, then the in- 

 terval between the records of the two pens driven by a short 

 and long circuit, diminished by the interval between the re- 

 cords when the variable pen is driven by a short circuit, will 

 exhibit the time occupied by the wave in traversing the di- 

 stance of 607 miles through the wires. 



I will now proceed to show the importance of fulfilling 

 strictly these three conditions: — 



1. To adjust to equality the intensity of the long and short 

 circuits operating through the receiving magnet upon the va- 

 riable pen. 



