The small sparks induced in a secondary conductor were the means used for detecting the electric forces in space. The primary conductor used may be most simply described as follows: Imagine a cylindrical brass body, 3 cm in diameter and 26 cm long, interrupted midway along its length by a spark-gap whose poles on either side are formed by spheres of 2 cm radius. The length of the conductor is approximately equal to the half wavelength of the corresponding oscillation in straight wires; from this we are at once able to estimate approximately the period of oscillation. With the aid of the circular secondary conductor it is possible to determine the position of the wave-front in the beam; this was found to be at right angles to the ray before and after reflection, so that in the reflection it was turned through 90 degrees.