I have made the following observations in the Natural Philosophy classroom of the United College, St Andrews, with the view of ascertaining whether the electromotive force required to cause a spark to pass between a small globe and a plate, or between a point and a plate, differs for the two kinds of electricity. Sir William Thomson suggested that I should apply to this question the method of measuring the electromotive force required to produce sparks, which I have described in papers already contributed to the Society (Trans. Roy. Soc. Edin. vol. xxviii. p. 633). It is a problem to which Faraday attached great importance. He says at sect. 1523, vol. 1, of his Experimental Researches in Electricity: “The results connected with the different conditions of positive and negative discharge will have a far greater influence on the philosophy of electrical science than we at present imagine, especially if, as I believe, they depend on the peculiarity and degree of polarised condition which the molecules of the dielectrics concerned acquire.” He records a great number of experiments on this subject in sections 1465–1525. He took sparks between a ball 0·25 inch in diameter and a ball 2 inches in diameter. When the large one was connected with a discharging train, the small one charged positively gave a much longer spark than when charged negatively; also the small ball charged negatively gave a brush more readily than When charged positively in relation to the effect produced by increasing the distance between tho two balls (sect. 1489).