Page:Scientific Memoirs, Vol. 1 (1837).djvu/527

Rh too narrow; at all events it ought to exceed half an inch. I have also made experiments with voltaic pairs arranged like the calorimotor of Hare, but there were reasons for rejecting these also. It is a very different thing to make an isolated observation, and to put in requisition the galvanic action for whole hours and days. It is in the latter case that for practical purposes measures are required, the necessity of which had not been before anticipated. It will be also necessary to reject the use of copper as a negative metal; the expense of employing silver, platina, or at least copper well plated with silver, gold, or platina, must be no obstacle. The solution of the copper in the sulphuric acid, however weak it may be, and its reduction into a metallic state, by the secondary effects of the nascent hydrogen, give rise to partial galvanic effects, by which the principal action is much affected, and to avoid which the greatest pains must be taken. In fact the motion of the magnetic apparatus was sometimes suddenly slackened or entirely interrupted, and on examining more closely I found that metallic particles of cementing copper or of iron had been deposited all along the pieces of wood interposed, or upon the bottom of the troughs, and thus formed a partial circuit. I shall speak of zinc hereafter. The following is the table of observations which I have made on the magnetic power of the horseshoe bar of iron above dsecribed.

The values given in the last column have been calculated according to the formula $$A = \frac$$, the constants of which were found by the method of the least squares. It is true that there are considerable differences between the observations of the same series, but there was no reason to choose those which agreed the best with each other, and to attribute the differences to an error of observation.

I have read in an extract from the memoir of the Abbé dal Negro inserted in the "Annali delle Sc., 1813 [1833?], Marzo e Aprile,