FACULTY OF APPLIED SCIENCE. 



ELECTRICAL MEASUREMENTS 



CHAPTER I 

 THE MEASUREMENT OF CURRENT 



Classes of Instruments. Electrical measuring; instruments 

 may be divided into two classes: 



1. Absolute instruments. 



2. Secondary instruments. 



An absolute instrument is one so designed and built that it 

 gives results expressed in the absolute, or < L ni of units. 



This implies that the numl>ers of turns in the coils and all the 

 dimensions which are electrically important have been deter- 

 mined, so that the factor which connects the force or the turning 

 moment acting on the movable member, and the numerical value 

 of the quantity under measurement can be calculated. 



A secondary instrument is one so constructed that the relation 

 between its indications and the quantity under measurement 

 must be established experimentally; that is. tin- instrument must 

 be calibrated. 



:nples of absolute instruments are the tangent galvan- 

 ometer and the Hayleigh current balance (see page 89). The 

 ordinary portable ammeters, voltmeters, and wattmeters are 

 examples of secondary instruments. 



Absolute instruments are not adapted for general use and are 



!y employed outside of such establishments as the Bureau 



ant Ian Is. the National Physical Laboratory or the Reichsan- 



stalt,. Their particular field of usefulness is in the determination 



the fundamental electrical c< , for example, the inter- 



national Mlllf" 



GALVANOMETERS 



The Tangent Galvanometer. This is an absolute instrument. 

 ists essentially of a circular coil of insulated wire, having 



