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PACIFIC CABLE COMMITTEL :

No. 5.

Put in by Mr. Lucas.

MEMORANDUM ON SPEED OF WORKING ON LONG SUBMARINE TELEGRAPH CABLES.

The working speed of a submarine telegraph cable depends on, and is inversely proportional to, the product of the total resistance of the conductor multiplied by the total inductive capacity of the core, so that, other things being equal, the speed varies inversely as the square of the length

100 lbs. copper

of the cable. Thus a core of 100 lbs. gutta-percha of a length of-

500 knots gives a speed of 120 words per minute.

1,000 2,000

2)

P

Or, on the other hand, if it requires-

נו

30

JI

71

*J

100 copper

100 gutta-percha

to give 30 words on 1,000 knots.

it follows that-

400 copper

PAPERS.

I handed to the chairman a series of slips of signals obtained at speeds varying from 43 letters per minute to 90 letters per minute on Lord Kelvin's Siphon Recorder through a Muir- head's artificial cable having the same amount of electrical retardation as the proposed cable of 3.600 knots in length, assuming that the core will have a capacity of about 1,530 microfarads and a conductor resistance of 8,120 ohma

By help of the series of Recorder slips I explained how the received signals became less and less definite and therefore more difficult to read as the speed of transmission increased.

Experts differ as to what is to be taken as safe signals for practical working.

The signals at the rate of 43 letters per minute were, I explained, plain enough for anyone who understands the Morse code, but good and experienced operators only can decipher those at the rate of 90 letters per minute.

According to my experience the average operator reads safely and readily signals such as those obtained at the rate of 76 letters per minute through the above amount of retardation.

2. In answer to question No. 971, I stated that the speed of cables depends upon the product of the total capacity of the core and the total resistance of the conductor, not upon the mere length

of the conductor.

I also stated that all results as to speed of transmission obtained by me on artificial cable in the laboratory have been exceeded in practical tests on cables with code traffic, and that therefore the answer given to question No. 958 may be considered as a perfectly safe one.

11

3. My answer to question No. 966 means that I have made improvements both in the "duplex method of working cables and in the construction of the apparatus required for successful automatic curb transmission on long cables. I am not the inventor of duplex telegraphy nor of automatic transmission.

PUBLIC RECORD OFFICE

Reference :--

TELEC.O. 885

חוי

and

400 gutta-percha will give 30 words on 2,000 knots.

900 copper 900 gutta-percha

will give 30 words on 3,000 knots.

Equal weights of copper and gutta-percha are taken in the above cores for the sake of comparison only.

In practice it is usual for the weights to be equal in small cores, for mechanical reasons, as a core with a very thin covering of gutta-percha would be difficult to manufacture with sufficient certainty of good insulation for a deep water submarine cable, but for cores larger than 150/150 it is more economical for the copper to exceed the gutta-percha in weight, the difference increasing proportionately with the size of the core, until in the largest aizes the copper may be as much as 50 or 60 per cent, heavier than the gutta-percha.

The expression "working speed " is understood to mean the speed at which ordinary traffic would be transmitted from day to day by trained clerks.

The attainable speed, or that which the most skilful clerk, working at the highest conceivable pressure, under favourable conditions, can reach, may be as much as 20 per cent. above the ordinary working speed.

In cases where the speed exceeds about 25 words per minute it is necessary to employ automatic sending instruments to develop the full capabilities of the cable, but as the speed is increased the size and definition of the recorder signals become less, and the limit of speed is reached when the signals begin to be illegible to a clerk well acquainted with their characteristics.

The unit of speed is a word of five letters, each letter being composed on the average of three siguals, but since the use of code words has become almost universal on submarine cable lines, the average length of a word has increased to about eight letters, so that in forming estimates of the carrying power of a cable with ordinary traffic the calculated working speed in words per minute must be reduced in the proportion of eight to five, or by 37.5 per cent.

And, further, it is found that to arrive at the actual number of paying words per minute that can be transmitted through a submarine cable, an allowance of about 40 per cent. must be made for prefixes, acknowledgments, vias, and other words which are sent free, so that in calculating the earning power or possible gross revenue that may be derived from a cable, not more than 38 per cent of the working speed, as expressed in words of five letters, should be taken into account.

For example, a cable which has a working speed of 30 words per minute of five letters can only be reckoned upon to transmit 11 to 12 ordinary paying code words per minute on a day's

average.

Duplexing will nearly double the carrying capacity of a cable, but only during the time that the business hours of the sources of traffic overlap.

No. 7.

Put in by Mr. Preece.

TABLE 1.

WORKING SPEED OF CABLES IN WORDS (OF EIGHT LETTERS) PER MINUTE.

Type of Cable.

Ratio of Copper to Gutta-percha.

Speed through

Speed Baross Pacific (3,628 Nautical Milen).*

2,000 Miles

Words

per

(Nautical),

ApDum.†

Theoretical.

Practical.

400

Standard

20.00

6-08

2.72

881,280

-

400

533

Silvertown proposal

22.21

6.70

3.00

972,000

365

500

Siemens proposal

20.35

6.30

2.82

913,700

320

650

Anglo-American (1894)

25.85

7.85

3.50

1,134,000

400

800

Post Office (revised)

35.20

10.70

4.82

1,661,500

550

940

47.00

14.30

6.40

2,169,600

940

Post Office (proposal)

No. 6.

Memorandum from Dr. Alexander Muirhead on his Evidence.

1. In answering question No. 958, I endeavoured to make clear to the Committee why experts may differ as to the practical or commercial working speed of any given cable.

* Allowing 10 per cent. slack.

† Possible number of words transmitted per annum when working duplex continuously, assuming 10 full working hours per

day and 300 days per annum,

W. H. PREECE.

November 25, 1896.

Gg 4

9

ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH—NOT TO

PUBLIC RECORD OFFICE, LONDON

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