132 Mr. E. L. Nichols on the Alternating 



Mr. Caldwell's first step, after having repeated the prelimi- 

 nary experiments of Archbold and Teeple, and verified their 

 statements, was to substitute a ball with a surface of platinum 

 for the brass ball used by them. The new ball withstood the 

 action of the arc no better than the old one had done. It 

 soon became covered with a black deposit, the growth of which 

 modified and vitiated the action of the apparatus. In experi- 

 menting with such a ball, the surface of which was still bright 

 and new, and with a point of the same metal, it was noticed 

 that within the critical distance, while the spark was passing 

 in both directions, there appeared to be two distinct paths 

 along which the discharge was taking place. One of these was 

 nearly in the line from the point to the ball, normal to the 

 surface of the latter, the other from the point in a direction 

 approximately at 45° with the common axis of the pointed rod 

 and ball. Upon increasing the distance until the discharge 

 entered the " one way " stage, the longer and oblique path of 

 flow vanished. In the revolving mirror the two classes of 

 sparks were easily identified. They were found to occur in 

 alternation with each other, the spark which followed the nor- 

 mal path being that which passed from ball to point, the other 

 that from point to ball. The images of the discharge from 

 the point disappeared as soon as the critical distance was 

 reached. In order to place the matter beyond all doubt, the 

 times of the discharge which followed the normal path were 

 determined by an ingenious method, quite independent of 

 that used by the first observers, and it was found that the 

 spark occurred always in that part of the cycle during which 

 the ball was positive. 



Mr. Caldwell's method of fixing the time of the discharge, 

 briefly stated, was as follows. An adjustable contact device, 

 similar to that used by Archbold and Teeble, was attached to 

 the shaft of the dynamo. A wire from one pole of a large 

 Holtz machine, driven by power, was carried to the neighbour- 

 hood of the ball and point, where two platinum terminals, 

 1 millim. apart, were set up. The wire was connected with 

 one of these, and a line was carried from the other to the con- 

 tact device. A wire from the latter to the remaining terminal 

 of the Holtz machine completed the circuit. Whenever the 

 brush made contact, a spark leaped between the platinum 

 terminals just described. By adjustment of the brush, the 

 spark could be made to appear at any desired instant in the 

 cycle of alternations of the dynamo. The platinum terminals 

 were placed so that the image of the spark in the revolving 

 mirror was seen side by side with that of the discharge 



