432 POPULAR LECTURES AND ADDRESSES. 



of the other great branch of electrical science, and 

 pointed to the subject of electro-magnetic measure- 

 ment, upon which I must now say a word or two. 



I think the principles of the mathematical theory 

 of the mutual interaction between one another, of 

 wires carrying electric currents, and again their 

 mutual action upon magnets, was fully laid down by 

 Ampere in his development of Orsted's discovery. 

 The working out of the accurate measurement of 

 currents, and generally of the system of measure- 

 ment founded on these principles, was done al- 

 together in Germany. The great work of Gauss 

 and Weber on terrestrial magnetism belongs 

 strictly to this subject. I believe Gauss first laid 

 down the system of absolute measurement for 

 magnetic force. The definitions and mathematical 

 theory of Poisson and Coulomb as to magnetic 

 polarity, and the theory of magnetic force founded 

 on it, was worked out practically by Gauss, and 

 made the foundation of the whole system of mag- 

 netic measurement now followed in our magnetic 

 observatories. This was an immense step in 

 science, and one of great importance, giving, not 



