December 26, 1918] 



NATURE 



323 



ELECTRIt AI. INSTRUMENTS. 

 Industrial Electrical Measuring Instruments. By 

 Kenelm Edgcumbe. Second edition, revised 

 and enlarged. Pp. xvi + 414. (London: Con- 

 stable and Co., Ltd., 1918.) Price 16s. net. 

 IN the student days of their career the majority 

 of electrical engineers are strongly attracted 

 towards the design and manufacture of large 

 machines. To design a io,ooo-h.p. dynamo is 

 found to be easier and far more exciting than 

 to invent some small improvement in a 

 measuring instrument. This is one of the reasons 

 which make it far more difficult to get — for 

 instance a manager for a telephone factory than 

 For a large machine shop. The former would 

 probably be offered a salary four times as large 

 as the latter, although the latter probably did 

 much better during his college course. It is 

 advisable, therefore, for students to remember 

 that there are certain drawbacks to following 

 what is for the moment the fashionable branch 

 of engineering to the neglect of much more 

 profitable branches. 



Mr. Edgcumbe's wide experience in the manu- 

 facturing and testing of instruments admirably 

 qualifies him for his task. Although there have 

 been no great changes introduced in the types of 

 instruments used since the first edition was pub- 

 lished, yet the continuous developments that have 

 taken place have made it necessary to rewrite the 

 whole book. Mathematical reasoning is avoided so 

 far as possible. One consequence is that the mathe- 

 matician who studies the work carefully will be 

 tempted to explore for himself numerous practical 

 and interesting problems. On p. 25, for instance, 

 there is a diagram of a volt-ammeter with an ohm 

 scale. Few mathematicians will be able to resist 

 the temptation of finding the equation to, and the 

 properties of, the curves which give the ohm 

 s. ale. Dials marked in this way are used in the 

 Board of Trade panel on the switchboard of a 

 central station. There is a great demand in 

 America for the analogous instruments which give 

 the volts, amperes, and watts on the same dial 

 They are useful in electric motor-cars. 

 The introductory chapters on "Errors and 

 r icy " are most helpful. A diagram (p. 9) 

 is given of a scale where the unavoidable error 

 of observation produces the same percentage in- 

 accuracy at all points of the scale. We take it 

 that if R is the reading and x the distance of the 

 pointer from the unit mark on the scale, then 

 R = n T gives the law according to which the scale 

 is divided, a being a constant. The divisions on 

 this scale, ordinarily called a logarithmic scale, 

 arc obviously much further apart at low 7 readings 

 than at high readings. We were surprised to 

 learn that the definition of the percentage error e 

 "usually adopted in this country" is given by 



True value = Reading (1 +<\'ioo). 

 We should have thought that the academic defini- 

 tion 



Reading = True value (1 c'ioo) 

 was more widely used. 



NO. 2565, VOL. I02] 



Amongst the instruments described are all types 

 of ammeters, voltmeters, wattmeters, frequency 

 tellers, synchronising devices, recording instru- 

 ments, speed indicators, oscillographs, pyrometers, 

 relays, rail-bond testers, etc. Descriptions are also 

 given of meggers, ohmers, ducters, graphers, etc. 

 rinse names are not schoolboy slang, but are 

 in everyday use by engineers. In the old days 

 anyone who could talk about an "earth" or a 

 "short-circuit " might rank as an electrician; now 

 he must be able to define the meaning of quaint 

 phrases like "milking booster." 



On p. 384, when describing instruments for 

 testing lightning conductors, the author says that 

 the earth should on no account exceed 10 ohms. 

 Is there any reason why the number 10 should be 

 chosen rather than, say, 1 or 100? The present 

 writer once measured the earth resistance of a 

 lightning conductor on a chimney-stack and found 

 it to be 70 ohms. He had the end dug up, and 

 found that the electrician had put a brick instead of 

 the usual copper plate at the end of the conductor. 

 This had been done twenty years previously, but 

 although the stack had been repeatedly struck by 

 lightning, no damage had been done. It would 

 probably have fulfilled its functions equally suc- 

 cessfully for another twentv years if the brick 

 had been left undisturbed. 



In conclusion, we congratulate the authoronthe 

 clearness of his descriptions and on the skill with 

 which he exposes the weak points in the design 

 of several well-known instruments. The diagrams 

 are worthy of the highest commendation, and the 

 printing is good. There are a few misprints. The 

 only serious one is on p. 183, where there are 

 two errors in the formula for the electrostatic 

 attraction between two plates. On p. 404 also 

 lines 4 and 5 from the bottom of the page should 

 be interchanged. A. Russell. 



OUR BOOKSHELF. 

 Modern Engineering Measuring Tools: A Hand- 

 book on Measuring and Precision Tools as used 

 in the Modern Tool-room and Engineering 

 Workshop. By Ernest Pull. Pp. viii+115. 

 (London : Crosbv Lockwood and Son, 1918.) 

 Price 45. 6d. net. 

 Prior to the outbreak of the war there were still 

 a large number of engineering firms in this 

 country which had not adopted systems of pre- 

 cision in the manufacture of component parts. 

 War conditions have altered this so far that there 

 must be very few firms now which have had no 

 experience in precision methods. It is to be hoped 

 that there will be no retrogression in this import- 

 ant matter now that the war is practically over. 



This hook is intended to help workmen to acquire 

 a practical acquaintance with the construction and 

 use of the commoner types of measuring- appli- 

 . as employed in tool-rooms and engineering 

 workshops. It includes sections dealing with the 

 use of micrometers, verniers, various special ap- 

 pliances such as depth-gauges, screw-thread 

 micrometers, etc., gauges and gauge systems, and 



