examples it is equal to 153 metres of telegraph wire. And it will be seen that as this resistance is increased or diminished, the figures given above may be singularly modified; for the value of R may then become less and less important, or may become more and more so, in the value of the denominator of the formula giving the intensity of the current. This, then, has caused many scientists to criticise this system. We might take, as a basis for the resistance R, the mean of the resistance of the batteries to be compared, but this would not advance matters much, and it could only be applied to short circuits. In this case we might start with the intensity I of the test battery and obtain the values of a and b of the battery composed according to the formulas a =

and b

2 Ir = E

2IR E which in the case of batteries of

Daniells and Bunsens, for a resistance R of 153 metres,

which bring us back to the conclusions previously arrived at, for then R is equal to the resistance of the test cell; but if R equals 1000, I becomes equal to 9.64; a = 3.2 and b = 3, and we see that instead of employing 20 Daniell cells, we have only to employ 9 to be equal to the Bunsen cell.

Cotton Covered.

Silk Covered.

TABLE II.

CALCULATED VALUES FOR THE WIRES OF ELECTRO-MAGNETS.*

32

34

28

32

24

31 full

20

28

16

27

12

Р

1

2

22

39

99

00014 00023 1 6428 2 6988 816 3 0000000529 · 00022·00033 1.5000 2 2500 330 6 0000001089 00027 00040 1 48142.1934 219.5 0000001600 00035 000481 3714 1.8796 130 6 0000002304 00040 00055 1.3750 1 8906 100.0 0000003025 25 bare00049 00065 1.3265 1 7582 66.60000004225 24 full 00058 000771.3275 1.7609 47.5 0000005929

23 bare 00060 001222 03334 1331 44 4 000001488 00070 00134 1.91433.6634 32 6.000001795 |00080 00146 1·8250 3·3306 25 0.000002132

00158 1.7555 3 0800 19.8 000002396 001721 7200 2.9584 16:0 000002958 001841 67272 7989 13.2 000003385 00196 1 6333 2 6687 11.1 000003842 002081 6000 2 5600 9 4 000004326 00220 1.57142.4670 8.2 000004840 00232 1 54662 3901 7.1.000005382 5.5 000006451

* These diameters of the wire, with or without its covering, are not exactly the results that would be obtained by actually measuring the wire; but these dimensions have been calculated from electro-magnets already made, whereby empty spaces in the coils and irregularities in the winding, which cannot be avoided, are brought into the values, and although the figures are in consequence not mathematically exact, the result is that we thus have much more correct data for practical purposes.

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A' Handbook of the Electro-Magnetic Telegraph. By A. E. LORING, a Practical Telegrapher.

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Electricity and the Electric Telegraph. By G. B. PRESCOTT.

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Terrestrial Magnetism and the Magnetism of Iron Ships. By

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Reports of the Committee on Electrical Standards appointed by the BRITISH ASSOCIATION. Revised by Sir W. THOMSON, Dr. J. P. JOULE, Professors CLERK MAXWELL and FLEEMING JENKIN. With a Report to the Royal Society on Units of Electrical Resistance, by Professor F. Jenkin. Edited by Professor FLEEMING JENKIN, F.R.S.

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Electric Transmission of Power: its Present Position and AdvanTAGES. BY PAGET HIGGS, LL.D., D.Sc., Telford Prizeman, and Associate of the Inst. C.E. CONTENTS:-Chap. I. Dynamo-Electric Machines-2. The Gramme Machine-3. The Brush Machine-4. The Wallace-Farmer and Siemens' Machines-5. Efficiency of Dynamo-Electric Machines-6. Practicability of Transmission of Power by Electricity-7. Efficiency of Coupled Machines-8. Comparative Efficiency of various Machines-9. Other Theoretical Considerations-10. Conclusions.

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The Telephone. A Lecture, entitled "Researches in Electro-Telephony,"
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