1 8 Mr. W. Sturgeon on the Tkenno-magfielistn of 



61. Let the same point be again heated in a similar manner, 

 and again place the cylinder over the needle with its heated 

 end north, but with the point of heat upwards. The north 

 end of the needle in this case will be deflected towards the 

 west, or in the opposite direction to that which it assumed in 

 the former experiment. This deflection of the needle indicates 

 the electric current to be flowing in the cold side of the cylin- 

 der from south to north, or in the opposite direction to that 

 in which it flows in the heated side. 



62. Heat the same point again : but instead of placing the 

 heated or opposite side of the cylinder over the needle, as in 

 the former cases, place one of those parts of the surfiice over 

 it, which is about 90° from the point of heat, still keeping the 

 cylinder parallel to the needle. In this position the latter is 

 scarcely affected ; and by a few trials a line will be found on 

 the surface of the cylinder, and nearly parallel to its axis, 

 which has no action whatever on the needle. This is one of 

 the neutral lines ; and by a few trials on the other side of the 

 point of heat, another neutral line will be discovered. These 

 lines are generally at about 90° from a line drawn from the 

 point of heat to the other end of the bar, and parallel to the 

 axis. 



63. This latter line is one of the lines of greatest energy ; 

 the corresponding line of greatest energy being parallel, but 

 on the opposite side of the cylinder [56). 



64<. Similar phaenomena will be displayed by making any 

 other part of the convex surface near to the ends of the cylin- 

 der the point of heat. The current uniformly flows over the 

 surface on the heated side from the point of heat, expands 

 into two distinct tides which sweep the surface of the metal, 

 and reuniting on the opposite side recurves into itself at the 

 heated point of the cylinder. 



G5. The general distribution of the electric force on the 

 surface of the cylinder by heating it as directed (60, 61, 62), 

 will be pretty accurately indicated by the arrows in fig. 13. 

 The cylinder is supposed to be divided into halves, and 

 its convex sides upwards, as in fig. 12 (56). The straight 

 arrows indicate the lines of greatest enei'gy; and the edges 

 ab, ab, which coincide when the halves ai'e replaced, are in 

 one of the neutral lines : the other neutral line is cd in the 

 centre of the figure. 



66. The thermo-magnetic energies can hardly ever be 

 traced more than four inches from the point of heat: they are, 

 liowever, excited to a certain extent by the slightest disturb- 

 ance of temperature near to either of the ends of the cylin- 

 der. 



67. When 



