402 



NATURE 



[July 19, 1917 



Charles Marquis Smith. Pp. xii + 373. (New 

 York : The Macmillan Co. ; London : Macmillan 

 and Co., Ltd., 1917.) Price 105. 6d. net. 



(4) A Laboratory Course of Practical Electricity 

 for Vocational Schools and Shop Classes. By 

 M. J. Archbold. Pp. ix + 211 +exp. 98. (New 

 York : The Macmillan Co. ; London : Macmillan 

 and Co., Ltd., 1916.) Price 55. net. 



(5) Electrical Measurements and Testing : Direct- 

 and Alternating-Current. By Chester L. 

 Dawes. Unpaged. (New York : John Wiley 

 and Sons, Inc.) Price 35. net. 



(6) Electrical Laboratory Course for Junior 

 Students. By Prof. Magnus Maclean. Pp. 

 120. (London: Blackie and Son, Ltd., 1916.) 

 Price 25. net. 



(i) qINCE the outbreak of the war there has 

 O been an urgent demand by our air., naval, 

 and military forces for information relating to 

 the range of searchlight projectors. We there- 

 fore welcome this translation of M. Jean Rey's 

 work. It is an open secret that during the course 

 of the war very great improvements have been 

 made in the manufacture of the carbons for pro- 

 jector lamps, and that they give a flux of light 

 for a given power consumption from three to five 

 times as great as that obtained from the old 

 carbons. 



The first electric searchlight was made by 

 Louis Sautter in 1867. In the early lamps the 

 carbons were arranged so that their axes were 

 inclined at an angle. The eflftciency was high, 

 but the regulation of the carbons was a difficult 

 and delicate operation. They are now always 

 arranged with the carbons horizontal. In the 

 first chapter the author gives an account of the 

 experiments he carried out in the laboratory of 

 the Sautter-Harle works in 1902. He proved 

 that for small arcs taking from 10 to 50 

 amperes the efficiency — ^that is, the ratio of the 

 luminous flux to the electric power — increases 

 with the current, but above 50 amperes the 

 efficiency diminishes as the current increases. For 

 instance, with a current of 250 amperes the 

 efficiency is 13 per cent, less than with a current 

 of 50 amperes. He also proved experimentally 

 that the diameter of the crater was proportional 

 to the square root of the product of the diameter 

 of the positive carbon and the current. In 

 chap. ii. formulae are given for the illumination 

 obtained with a specified reflector. From 

 these it appears that, neglecting absorption, the 

 illumination at a given distance is proportional 

 to the square of the focal length of the reflector, 

 and is inversely proportional to the square of the 

 diameter of the crater. The efficiency of electric 

 searchlights is next considered, the losses due to 

 the front glass, the shadow losses due to the 

 lamp, and the losses due to the flashing shutters 

 being taken into account. 



Blondel's law for the range of a searchlight is 

 fully explained, and values are given for the co- 

 efficient of atmospheric transparency. When the 

 humidity of the air is great the coefficient is per- 

 ceptibly reduced, and when the air contains 

 NO. 2490, VOL. 99] 



particles of dust the coefficient is very appreciably 

 diminished. In the last chapter the difficult 

 problem of the influence of visual acuity on the 

 range is discussed, and references are given to 

 Blondel's work. 



Numerous interesting phenomena are men- 

 tioned. For example, when convoys, pioneers 

 making trenches, aviators, etc., are caught by 

 the beam of a searchlight at a great distance they 

 generally imagine that they must be visible to the 

 enemy and so take cover. It is pointed out that 

 in many cases the enemy would be quite unable 

 to see them. The work will prove of great value 

 to engineer officers. We have noticed one or two 

 misprints in some of the mathematical equations, 

 but the corrections are obvious. 



(2) A knowledge of easy methods of measuring 

 and of rapid methods of calculating with high 

 accuracy the inductance of currents and the 

 capacity of conductors is of great value in radio- 

 telegraphy. On these subjects, therefore, a great 

 deal of experimental ingenuity and mathematical 

 labour have recently been expended. Mr. 

 Nottage begins by giving formulae for inductance 

 and capacity, and he illustrates their use by 

 numerous numerical examples. He then 

 describes experimental methods of measuring 

 these quantities, and finally gives brief descrip- 

 tions of the appliances now used in making these 

 measurements. Extensive use has been made of 

 papers which have recently been read to the 

 London Physical Society and to the Institution 

 of Electrical Engineers. 



As the author is writing for the benefit of 

 physicists and engineers, the mathematical proofs 

 of the formulae, which are as a rule long and 

 tedious, have been omitted. The publication of 

 a formula without proof, however, has its draw- 

 backs. The user of a formula obtained in this 

 way is at the mercy of misprints. As the limita- 

 tions of the formulae also are not given in all 

 cases, he may easily fall into error. In chap, ii., 

 formula (11), p. 41, we notice that in the formula 

 for the joint capacity of two spheres -3863 has 

 been printed instead of 1-3863. The limitations 

 of the formula for the inductance of a rectangle 

 are not stated. Rayleigh's formula for the 

 inductance of a concentric main, which is perhaps 

 the most important of them all, has been left out 

 altogether. 



On p. 47 a formula (18) is given for the average 

 potential of a single straight wire. We are quite 

 unable to make sense of the formula. It is stated 

 to be only approximate, but then the "accurate 

 expression" (19) is also given; and finally, we 

 are told that for all practical purposes the dif- 

 ference between the two formulae is negligible. 

 Doubtless also to the same degree of accuracy the 

 approximate formula for the capacity of a wire 

 deduced from that of a prolate spheroid by mak- 

 ing the equatorial axis very long will agree with 

 either. Unfortunately, the exact solution is not 

 known. We therefore have to pass over the next 

 fourteen pages, as we cannot understand them. 

 Heaviside has shown how to calculate the 

 capacity and Inductance of horizontal antennae 



