348 Mr. J. D. Chorlton. Examination of some of the [Mar. 19, 



The second electric current meter was constructed some years later, 

 and while its design is almost exactly similar to the one just described, 

 its construction is in every way more finished and elaborate, and as 

 it is considerably smaller than the old balance (exactly jths the size), 

 it is more convenient to work with. 



A very complete description of this balance is given in the ' Col- 

 lected Papers,' vol. 1, pp. 584 — 589, it is, therefore, unnecessary to 

 describe it at length here, I will only mention that a great improve- 

 ment is made by the use of copper tape instead of copper wire in the 

 coils, the flat tape does not take up nearly as much space as the wire, 

 so that two coils made of tape will contain many more revolutions 

 and consequently will attract each other more strongly than two 

 coils of the same diameter made of wire. 



This may be seen from the following determination of the constant 

 by means of the Kelvin balance. 



Current c. 



Weight w. 





Constant. 



4'468 amperes 



49'98 grams 



7-067 



0-6322 ampere 



3'880 „ 



37-72 „ 



6-140 



0-6319 



3-468 „ 



30-13 „ 



5-489 



0-6318 



3-159 „ 



25-02 „ 



5-002 



0-6315 



2-638 „ 



17-47 „ 



4-178 



0-6314 



2-284 „ 



13-10 „ 



3-019 



0-6313 



2-019 „ 



1024 „ 



3-198 



06313 



Here again the steady decrease of constant with decrease of current 

 is noticeable. 



From the above table it will be seen that when 3 amperes are 

 passing, the weight needed to restore equilibrium is about 25 grams, 

 while it will be remembered that in the former balance only 10 grams 

 were needed, and as in this case also the weights could be adjusted 

 to 5 milligrams or less, 3 amperes may be measured with this balance 

 correctly to 1 part in 10,000. 



Joule's determination of the value of J by the electrical method 

 was undertaken at the request of the Committee on Electric 

 Standards appointed by the British Association ; the result he 

 obtained was published in the B. A. Report, Dundee, 1887. 



He experienced a great deal of trouble in measuring the current 

 because of the difficulty he found in determining the exact value of 

 the earth's horizontal magnetic force at the time and place of the 

 experiment. 



The arrangement he finally adopted was to pass the current 

 through a tangent galvanomter and then through an electric current 

 meter placed in series, with it, the current being measured by the 

 galvanometer and the current meter being used to determine the 

 earth's horizontal magnetic intensity ; this was effected as follows : — 



