494 Prof. J. A. Fleming. On an Instrument for the [Jan. 26, 



wire included, and the oscillation constant when the circular wire 

 was not included. It was found that 0' = 3*5 and O = 3-0. Whence 



a value in fairly close agreement with the calculated value, considering 

 that the inductance is less than 2 '5 microhenrys. 



In this manner it is possible to determine the inductance of a foot 

 or two of coiled copper wire for high frequency currents with fair 

 accuracy. The oscillation constant of an instrument of the above form 

 for various positions of the outer jacket or inner jacket, according to 

 the form used, can best be determined directly by means of a standard 

 wave-meter, such as that described by the Author in a paper to the 

 British Association at Cambridge, September, 1904.* 



In this last mentioned arrangement, a long ebonite tube is wound 

 over uniformly with a fine silk-covered copper wire in closely adjacent 

 turns and in one layer. The capacity and inductance per unit of 

 length (c and /) of this long helix must then be determined by known 

 methods, and from this measurement we can determine the velocity of 

 propagation of an electric wave along the helix, for it is equal to the 

 reciprocal of the square root of the product of the capacity and 

 inductance per unit of length of the helix. If we form an oscillatory 

 circuit (see fig. 5), consisting of a condenser, C, a variable inductance, 

 L, and a spark gap, S, the variable inductance including a length 

 of straight wire AB, which can be placed parallel with, and close 

 to, the copper bar of the form of wave-meter described in this 

 paper, then we can bring its oscillation constant into agreement 

 with the oscillation constant of the circuit formed of the variable 

 inductance and condenser. In order to determine the value of this 

 oscillation constant we connect the long helix above described to one 

 terminal of the condenser of the oscillating circuit above described, 

 one spark ball being to earth. The arrangement must be as shown in 

 fig. 5. The long helix of insulated wire HH is then provided with a 

 sliding metal saddle D, which can be connected to the earth E, and 

 this saddle is moved along the helix until a position is found such 

 that by means of a neon or other sensitive vacuum tube V, we can 

 detect a node of potential half-way between the saddle and the point 

 of attachment of the helix to the oscillating circuit, the saddle itself 

 being connected to the earth will also be a node of potential. Hence 

 the distance between the saddle and the end of the helix attached to 

 the oscillating circuit is equal to one wave-length of the wave travelling 

 along the helix. 



* See J. A.Fleming, " On the Propagation of Electrie Waves along Spiral Wires, 

 and on an Appliance for Measuring the Length of Waves used in Wireless Tele- 

 graphy," ' Phil. Mag.,' October, 1904. 



