April 11, 1913] 



SCIENCE 



563 



in the adjustment of resistance coils is de- 

 scribed; and valuable suggestions given in 

 regard to the selection and use of bridges and 

 auxiliary apparatus. 



Methods and apparatus for measuring low 

 resistances and calculation of conductivities 

 are treated fully, as are also the methods and 

 apparatus for measuring high resistance and 

 the insulation resistance of cables. A chapter 

 is devoted to the measurement of resistance- 

 containing electromotive forces, insulation re- 

 sistance with power on, resistances of batteries 

 and electrolytes. Some of the methods de- 

 scribed here were developed by the author. 



A description is given of the author's dy- 

 namometer method for measuring the resist- 

 ance of an inductive conductor to alternating 

 current. A non-inductive resistance is ad- 

 justed so that the power dissipated in it is 

 equal to the power dissipated in the inductive 

 resistance as shown by the dynamometer. 

 The two resistances being in series carry the 

 same current and therefore have the same re- 

 sistance. 



The methods and apparatus used in locating 

 faults are fully described. Here again some 

 of the methods were developed by the author. 



One of the important applications of elec- 

 trical methods to other physical measure- 

 ments is that of the measurement of tempera- 

 ture or temperature changes by means of the 

 resistance thermometer. Apparatus intended 

 for use between and 100° C. should be ca- 

 pable of indicating small changes in tempera- 

 ture with certainty to .001° C. This means 

 that changes in the resistance of the ther- 

 mometer must be measured to better than 3J 

 parts per million of its total resistance. 

 Therefore, careful attention must be given to 

 the compensation for the various changes in 

 resistance of leads, changes in resistance of 

 contacts, etc. As the resistance thermometer 

 must have a small heat capacity to follow tem- 

 perature changes quickly and as it has a large 

 temperature coeiEcient, the power supplied by 

 the test current must be kept very small in 

 comparison with that which might be used 

 in the measurement of ordinary manganin 

 coils of the same resistance. For this reason 



a sensitive galvanometer is required and care 

 must be taken to correct for the thermo-electro- 

 motive forces, practically always large enough 

 to introduce an error of a few thousandths of 

 a degree. In discussing resistance thermom- 

 eters the author apparently fails to appreciate 

 the importance of some of these matters, for 

 the arrangement which he is inclined to be- 

 lieve " offers more advantages than any other " 

 if used in ordinary calorimetric work should 

 hardly be expected to give satisfactory results. 

 Much difficulty would be experienced in an ac- 

 curate compensation for the changes in the 

 resistance of the galvanometer windings. 

 Then unless the galvanometer were more sen- 

 sitive than the one which the author states 

 " is amply sensitive for the purpose " we 

 should hardly expect an accuracy better than 

 that obtainable with a good mercury in glass 

 thermometer. The galvanometer should be at 

 least a hundred and preferably over a thou- 

 sand times more sensitive. 



Under the heading " Instruments Used for 

 Measuring Resistances " the reader will find 

 much in the nature of good advice both to the 

 intending purchaser and user of such appa- 

 ratus. 



In the consideration of deflection instru- 

 ments and galvanometers attention is called 

 to what constitutes desirable qualities in 

 anmeters, voltmeters, pointer galvanometers 

 and mirror galvanometers. Instruments de- 

 signed by the author and constructed under 

 his supervision are described. The author's 

 method of comparing galvanometers is given 

 fully, together with a table giving constants 

 of sixteen galvanometers. In this connection 

 no reference is made to the important papers 

 of White and Jaeger. 



In an appendix a few mathematical formu- 

 las and tables, and physical constants are 

 given. 



Practically the entire field of resistance 

 measurements is covered and most of the 

 methods used are well described. However it 

 is assumed that a .01 or even a .1 per cent, is 

 a high accuracy and sufiicient for all but very 

 exceptional cases. The book will be welcomed 

 by almost every one who uses resistance appa- 



