ELECTRICAL MEASUREMENTS. 283 



length of the current which acts upon the magnet as well 

 as its distance from it; and in this way we have the 

 measurement complete. 



The apparatus which may be used for applying a 

 measurement upon these principles may receive very 

 various forms. This is one of an essentially very simple 

 kind. It has the advantage that it can be employed to 

 measure currents of very different degrees of strength. 

 If we have a very strong current it may deflect a magnet 

 so much that we could not readily deduce the amount of 

 force exerted. If there is too great a force applied to any 

 particular instrument its indications are not so certain. 

 We have a familiar example of this in the case of a spring 

 balance. It will indicate, perhaps, very accurately, weights 

 up to a certain amount, but if you put on too great a weight 

 then the indication becomes uncertain. So any particular 

 measuring instrument acts best for quantities which lie 

 within a certain range ; and it generally requires a different 

 instrument or different adjustment of the same instrument 

 to measure quantities differing greatly from one another. 

 I may just point out the way in which this particular 

 instrument may be modified to suit stronger currents than 

 those which act upon it as now arranged ; that is by dis- 

 placing the circular conductor from the magnet. The 

 magnet is not at the centre of the circle, but each portion 

 of the wire, instead of surrounding the magnet, is on the 

 surface of a cone, of which the magnet is at the apex, and 

 the further we move it in this direction the smaller becomes 

 the effect of each portion of the wire, and therefore the 

 stronger the current required to give a definite deflection. 

 Here is another form of apparatus for the same purpose. 

 It is essentially the same instrument although smaller in 

 size and in various ways differently arranged. There is 

 a point at the centre for carrying a small magnet, a gradu- 

 ated circle to show how much the magnet is deflected, and 

 a vertical circle in which the current is to pass. Most of 

 you are aware that in using an instrument such as either 

 of these, it must be placed in a certain position ; that the 

 plane containing the circular conductor must coincide with 

 what is called the plane of the magnetic meridian. The 

 circle must in the first place be vertical, and it must also 

 coincide with the plane of the earth's magnetic force. 



