NATURAL PHILOSOPHY. 155 



EXPERIMENTS ON THE ATLANTIC TELEGRAPH CABLES. 



The report of the Coast Survey Expedition of 1866, regarding 

 electrical measurements made on the cables of 1865 and 1866, 

 has been published in the Smithsonian " Contributions." The 

 following is a summary of Dr. Gould's conclusions taken from 

 the " Journal of the Franklin Institute : " 



Four leading questions were to be answered by the experi- 

 ments. 1. The character of the agency which gives the tele- 

 graphic signal upon the closing or interruption of the galvanic 

 circuit, and the route by which its transmission is effected. 



2. The influence exerted upon the conductor by using the 

 earth as part of the circuit, or by placing the complete circuit in 

 electrical communication with the earth. 



3. The extent to which the velocity of propagation of the sig- 

 nals is dependent upon the intensity of the electromotive force, 

 and upon the resistance of the conductor. 



4. The equality or difference in speed of the signals from the 

 positive and from the negative electrode, when the other is con- 

 nected with the earth ; as also the relative velocity of signals 

 given by completing and by interrupting the circuit. 



' The length of the cable of 1865 is 2,186 statute miles, and that 

 of the cable of 1866 is 2,134 statute miles. Each conductor is con- 

 structed of 6 copper wires twisted round a seventh one, and has 

 a diameter of one hundred and forty-seven one thousandth 

 inches. The conducting power of the wire is 93.1 for the cable 

 of 1865, and 94.6 for that of 1866, the conducting power of pure 

 copper being 100. The cable of 1865 gave a resistance of 4.01 

 ohms per knot, the " insulation " or resistance of the coating 

 being 2,945 megohms per knot, and the electrostatic capacity 

 0.3535 farad to the knot. That of 1866 gave a resistance of 3.89 

 ohms per knot, and an insulation of 2,437 megohms, the electro- 

 static capacity being the same as in the cable of 1865. The total 

 resistance to conduction in the first cable is, then, about 7,650 

 ohms, the total resistance of the insulator 1,505,000 ohms, 

 and the total electrostatic capacity about 670.4 farads. In 

 the second cable, the total resistance is about 7,270 ohms, the 

 total insulation 1,316,000 ohms, and the total electrostatic capac- 

 ity 654.5 farads. 



"The battery employed by t the telegraph company was Minot- 

 ti's, a modified form of DanielFs. Mr. Farmer estimates that 

 after the full strength of the current is developed, one cell should 

 give upon one cable with earth-connection about 110 farads 

 per second. 



Several series of experiments were made with varying battery 

 power, and different arrangements of the connections, in some 

 using the earth circuit, in others not using it, sending both posi- 

 tive and negative signals. 



The first question to be investigated is, whether the posi- 

 tive and negative signals were transmitted with the same veloc- 

 ity. A comparison of the records of the same signals at the two 

 stations decides this, without the necessity of knowing the abso- 



