Prof. Ayrton and Mr. Mather on Galvanometers. oV) 



Another comparison can be made in the case of 

 Mr. McKittrick's oscillograph, which has a needle of almost 

 microscopic dimensions. In this instrument / = 0*053, and 

 we see from Table VII., that we might possibly get 150,000 

 divisions per microampere when the resistance is one ohm, 

 period 10 seconds, and scale -distance equals 1000 scale- 

 divisions. Line 4 of Table II. gives the values 60,000 and 

 105,000 for D/r* and D/r* respectively. These numbers 

 approximate more closely to the theoretical values than those 

 for the Mudford galvanometer previously referred to, the 

 probable reason being that the needle of Mr. McKittrick's 

 instrument consists of a piece of soft iron placed in a very 

 strong magnetic field. The specific magnetism would therefore 

 be much greater than that of short permanent magnets in a 

 weak field such as used in controlling the needle of the 

 Mudford galvanometer. In all probability the value of a for 

 Mr. McKittrick's instrument would be two or three times as 

 great as the value used in calculating Table VII. 



Section C. 

 Long versus Short Period Galvanometers for Zero Methods. 



For rapidity and accuracy of working a short-period 

 instrument is certainly better than a long-period one, providing 

 its sensitiveness is as great. It is interesting, however, to 

 inquire whether, with the same galvanometer, it is more 

 expeditious to use a strong or a weak control, provided the 

 sentitiveness when strongly controlled is sufficient for the 

 purpose. For although the spot will move to its maximum 

 elongation, corresponding with a given want of balance, more 

 quickly when strongly controlled, that elongation will be less 

 than when a weak control is used. Hence it is possible that a 

 given small displacement of the spot from the zero position 

 may occur in a shorter time when the control is weak than 

 when it is strong, because such displacement is then a smaller 

 fraction of the whole. 



First applying general reasoning to the problem, we may 

 notice that the deflecting couple produced by a given want of 

 balance is initially independent of the control, and as the 

 inertia of the moving system is constant, the acceleration at the 

 beginning of the motion is the same whatever the control. 

 If / be this acceleration the space traversed by the spot in a 

 short time t is ^ft 2 , and this is therefore independent of the 

 control, consequently for small displacements strong or weak 

 control makes no difference in the time it takes to see that a 

 want of balance exists. 



