ELECTRICAL MEASUREMENTS. 



TWO LECTURES. 



BY PROF. CAREY FOSTER. 



LECTURE I. 



UNDER the circumstances of these Lectures I think it will 

 probably "be most useful, if, instead of attempting to 

 describe the details of the construction or use of special 

 instruments, I try to explain as well as I can in the time 

 available, the principles upon which the use of instruments 

 of any general class depends. You are aware that the 

 subject of the present lecture, and the one to follow next 

 week is Electrical Measurement, and I shall to-day speak of 

 measurements which have relation to Statical Electricity or 

 electrostatic measurements. Here the magnitudes which 

 have to be measured are the Quantity of electricity, the 

 Potential of the electricity, and the electrical Capacity of the 

 bodies which may be charged. Then we have also the Energy 

 represented by a quantity of accumulated electricity or a given 

 electric charge. These are the fundamental magnitudes with 

 which we have to deal in electrostatics ; but there are various 

 secondary matters which may be made a subject of measure- 

 ment, for instance electrical Density, which is the quantity of 

 electricity per unit of volume. If we have any electrified 

 substance the quantity of electricity in unit of volume of that 

 body is the density of the charge ; if we have a charged con- 

 ductor, then, as is well known, the electric charge is confined 



