140 Scientific Proceedings, Royal Dublin Society. 



In the upper set of curves, the lowest one gives the graph of the values 

 in column 2, Table I, that is when the steel laminations were in contact, and 

 the highest curve the values in column 5, Table I, that is when the lamina- 

 tions were separated by a piece of paper 0'06 cm. thick, or of the same 

 thickness as the steel plates ; and the two intermediate curves give the graphs 

 of the values in columns 3 and 4 of Table I when the thickness of the 

 separating paper was 0'015 cm. and O03 cm. respectively. Each one of these 

 curves seems to be a parabola ; the equation of the lower curve is x % = 25y, 

 and its axis of figure when produced meets the axis of abscissa; at the point 

 corresponding to 5 laminations instead of at 9, that is at an angle of 

 85 degrees instead of 90 degrees. It will be seen that as the cross-section 

 of the compound magnet grows from a rectangle to a square the distance 

 between the poles decreases, and as the section further grows from a square 

 to a rectangle the distance between the poles again increases, that is, the poles 

 are at the least distance apart when the cross-section of the compound 

 magnet is very approximately a square, whether the magnet is built up of 

 steel or steel and paper combined. 



From the lower curve of magnetic moments, it will be seen that when there 

 are nine laminations the magnetic moment is 455 units, and when the number 

 of laminations is eighteen the moment is 560 units, or an increase of only 23 per 

 cent. From the upper curve, when there are five laminations each separated 

 by paper of thickness 0'06 cm. the moment is 370 units, and with ten lamina- 

 tions the moment is 525 units, an increase of 42 per cent., whilst with twenty 

 laminations the moment is 635 units, a further increase of about 21 per cent. 

 It would appear, therefore, that the best magnet to use, with the given steel 

 plates, would be one made up of nine laminations in contact, that is, when 

 the cross-section of the magnet is practically a square and the poles at the 

 minimum distance apart. 



