316 Profs. W. E. Ayrton and J. Perry. [Feb. 14, 



and since SC is great in comparison with nnity for such currents as 

 we wish to measure, we have 



K S S** 



that is, equal divisions of the scale correspond with equal additions to 

 the strength of the current except close to the zero, and we usually do 

 not graduate these instruments within 5° of the zero. 



Shielded Measuring Instruments. — When it is desired to use the instru- 

 ment close to a dynamo machine or electromotor in action, we have 

 adopted a different and somewhat special form of construction, which 

 is shown in fig. 8, by means of which the instrument is to a great 

 extent shielded from even powerful external magnetic fields. In this 

 instrument the electromagnet consists of a hollow core, part of 

 which, BB, is of charcoal iron, and part, DE, of brass or other non- 

 magnetic metal. The outside tube, CO, and the plates, XX, top and 

 bottom, are also of charcoal iron. The space EE is filled with insu- 

 lated wire in electrical connexion with the terminal, so that when a 

 current is sent through the instrument an intense magnetic field is 



