demonstrating a limit to the Magnetizability of Iron. 449 



It was my desire to make my experiments as exact as possible ; 

 and as I wish the relation of them to be clear and definite/ 1 

 shall begin with some observations on the measure of current 

 electricity indicated by my galvanometer. I iuiii^rb do/rr riii 'io- 



* * ^ *««''*. • ' 



The galvanometer of which I made use in the last series of 

 researches was connected with an electrolytic apparatus furnished 

 with very fine platina wires. Voltaic currents of varied intensity 

 were then conducted through the circuit which included the two 

 instruments^ the circuit being broken at the end of two or three 

 minutes in each case_, and the hydrogen measured in a graduated 

 glass tube. The mean of ten trials gave 0*76 of a grain as the 

 quantity of water decomposed during each hour by the electrical 

 current indicated by the unit of my former quantity num- 

 bers. Hence the current indicated by 11-8 of these last would 

 decompose nine grains, or one equivalent of water per hour. 

 This current I propose to call a degree in the present paper. 

 The dimensions of the single coil of the above galvanometer are 

 12 inches by Q, and the deviation of its needle for one degree, 

 34° of the graduated card. From these data we may easily 

 calculate the value of the indications of any similar instrument, 

 bearing in mind that the electro-dynamic force produced by a 

 constant current of electricity is directly as the number of coils 

 and inversely as their linear dimensions. 



The quantities of current electricity which were brought into 

 play in the subsequent experiments were frequently so great, that 

 the needle of the above galvanometer would have been brought 

 almost to a right angle with the plane of the coil, if subjected to 

 their influence. 1 therefore devised a new mea- 

 sure, which I flatter myself will prove of greater 

 service in some cases than the instrument pro- 

 posed for the same purpose by Mr. Iremonger*. . 

 The plan of my instrument is represented by the 

 accompanying figure, in which cc is a rod of cop- 

 per bent double, and fastened firmly to a strong 

 wooden frame ; m is a magnetized cylindrical bar 

 of steel, one foot long and half an inch in dia- 

 meter, supported slightly above the centre of 

 gravity (like an ordinary balance-beam) by knife- 

 edges resting on hard concave surfaces of steel. 

 A scale 5 is attached to one end of the magnet for 

 the purpose of receiving the weights by which the 

 electrical currents are measured. Lastly, rr is a 

 rest which the magnet just touches when at zero.' 



In using this instrument, it is merely neces-' 

 sary to adjust the magnet to zero, either by 



n 



VI 



J 



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)l) bii;i iif-j(i 



rd LabfliMj 



Annals of Electricity, vol. iii. p. 413, 414. 



