demonstrating a limit to the Magnetis ability of Iron. 311 



to use a galvanometer the indications of which might be depended 

 upon. 



The accompanying figure re- oc 

 presents the form of galvanometer 

 I have employed. The needle n 

 is 2 inches long. The wire is 10 

 feet long, and y^gth of an inch in 

 diameter : it is disposed in four 

 circumvolutions, mercury cups 

 being placed at the parts marked 

 tti by c, d, e. The coils cross one another at x, x, but in every 

 other part they are in the same plane. By this contrivance the 

 forces of the several coils are made equal to one another, or, as 

 it would seem, not perceptibly different. 



The process of graduation was conducted in the following 

 manner : — The electricity of a constant battery was first passed 

 through each of the coils in succession, and the deflection of the 

 needle was observed to be the same in each case. A current of 

 a certain intensity was then passed from a to b^ a to c, a to d, 

 and a to ^, taking care to decrease the resistance of the battery 

 wires in proportion as the length of that part of the galvano- 

 meter wire through which the current passed was increased, and 

 I marked the several deflections of the needle on the card of the 

 . instrument 1, 3, 3, 4. I then increased the power of the battery 

 until the needle stood at the mark 2, when the current passed 

 from a to b; the former process was then repeated, and I thus 

 observed the quantities 2, 4, 6 and 8 ; and going on in the same 

 manner, I had the card of the galvanometer marked with the 

 numbers 1, 2, 3, 4, 6, 8, 9, 12, 16, &c. In using the galvano- 

 meter the current is passed from a to 6, and the above numbers , 

 indicate absolute quantities of current electricity. 



In order to obtain a definite idea of the quantities indicated 

 by my galvanometer, I took a diluted acid, consisting of ten 

 parts water and one of sulphuric acid, spec. grav. 1*8, and passed 

 through it a current which deflected the needle to the mark 1. 

 In seven minutes 0*62 of a cubic inch of the mixed gases was 

 produced. The electrodes employed in the above experiment 

 were pointed platina wires I'l inch asunder. 



The electro-magnets used first were those described in my last 

 communication. They are straight and square, 7 inches long, 

 and wound with twenty-two feet of copper wire y^^th of an inch 

 in diameter. Five of them were constructed of bar iron, and 

 five corresponding ones of square iron wire. The sections of 

 those marked No. I. are j^ths of an inch square ; a dimension 

 which is successively increased in Nos. II. III. and IV. until 

 No. v., which are jyths of an inch square. 



