650 . Prof. Pollock: Comparison of Periods of 
tions with a closed ellipse using the longitudinal detector (see 
infra). In this case the observations near the maximum are 
taken with ellipses differing in perimeter by 10 cms. 
The maximum is much more definitely marked in the case 
of open circles than in circuits of other shapes, and here a 
want of tune between the condenser and rectangular circuits 
seems to have less effect in altering the character of the curve 
of the observations than in other cases. It has been found a 
more difficult matter to get a series of accordant observations 
with closed circuits than with open ones. 
The Sparks in the Condenser Circuit.—Practically the only 
difficulty in connexion with the determination of numerical 
relationships in the case of Hertzian waves of small wave- 
length, lies with the spark which discharges the condenser. 
Many forms of detector are completely reliable. On the 
other hand, the character of the sparks is so variable and the 
conditions which determine it at present so obscure, that 
chance seems to enter largely into these investigations. 
Many spark-knobs have had to be tried in some cases in this 
research before the observations became quite accordant. 
To obtain a more uniform effect, a series of sparks may 
be taken for each observation, a method which has been used 
by many experimenters. As is well-known, however, only 
a limited number of sparks can pass between the knobs before 
the character changes owing to the deterioration of the dis- 
charge surfaces. In the present case, after trial, it was on 
the whole considered best to discharge the condenser once 
for each observation. 
The condenser was charged from the secondary of a small 
induction-coil. The primary circuit of the coil was closed 
and opened by two keys worked by a heavy pendulum, the 
interruption taking place very quickly. The circuit was 
opened in air, the gap being short-circuited by the condenser 
of the coil. A variable resistance was inserted in the primary 
circuit, and adjusted so that when the spark-knobs were set, 
the difference of potential established on working the coil 
was just sufficient to break down the dielectric. 
If the spark-gap is watched through a lens, as in these 
experiments for each discharge, it is seen that successive 
sparks jump across the gap from different points of the knobs. 
Their character may be either round, long, or irregular, either 
large or small, single or double, scarcely ever quite the same 
for two sparks together. The demagnetization of the detector 
has been found to so greatly depend on the path of the spark 
and on its character, that it was useless to retain any observa- 
tions except in those cases where the sparks passed across the 
centre of the gap, and where the character was as far as could be 
