330 Mr. J, N. Hearder on a new Instrument for Registering 



with an increased velocity due to the attraction of the inducing 

 surfaces upon each other. 



In order to examine the phsenomena accompanying the gradual 

 change in the electrical capacity of the terminals^ I instituted the 

 following experiment, 



I coated with tinfoil two large flat discs of wood, each exposing 

 an ai'ea on one side of about 5 square feet ; one of these was in- 

 sulated and placed upon the table with its coated side upwards ; 

 the other was suspended from a frame by silk strings passing- 

 over pulleys and attached to a counterbalancing weight, and was 

 placed 3 or 4 feet above the first one and with its coated side 

 parallel to it. I next connected with the terminals of one of 

 my large coils a tliermo-electrometer and a discharging electro- 

 meter, adjusted so as to cause a current of sparks of 0"8 of an 

 inch in length to pass through the formei-, but no heating effect 

 was produced. Without disturbing this arrangement, the ter- 

 minals of the coil were now respectively connected additionally 

 with the two coated discs before mentioned, by which their con- 

 ducting surfaces were virtually increased ; the sparks in the dis- 

 charging interval instantly became brighter and louder, but no 

 effect was yet produced upon the electrometer. The upper disc 

 was now gradually depressed; and as it descended, and the in- 

 ductive action between it and the lower one increased, the dis- 

 charges gradually became more sonorous. When within about 

 3 inches of each othei', they began to assume the character of 

 the Leyden discharge, and the thermometer showed a small rise 

 of temperature ; and as the discs approached still nearer, and 

 their inductive action upon each other still further increased, the 

 thermometer rose to 15°, aud the sparks became as vivid as 

 those from a small Leyden jar. On raising or depressing the 

 upper disc, corresponding changes in the character of the dis- 

 charges and their thermal effects were produced. The surfaces 

 here opposed were each 5 square feet, and their effect iipou the 

 electrometer when at | of an inch from each other, was just equal 

 to a Leyden jar of moderate thickness containing about a quarter 

 of a square foot of surface. Thus it would appear, that in order 

 to obtain the effect of a jar of 3 or 4 square feet, it would require 

 such discs to possess each a surface of at least 60 to 80 square 

 feet, and to induce upon each other at a distance of | of an 

 inch ; and I am not quite sure that even this would i.)e sufficient 

 to develope the full effect of a single wave. Some idea may be 

 thus formed of the extraordinary quantity of electricity which 

 here appears to be set in motion by each interru])tion of the 

 battery current ; and when it is considered that a simple increase 

 in the length of the induced wire, without interfering in any 

 way with the battery or the primary wire, is sufficient to produce 



