RECENT PROGRESS IN PHYSICAL SCIENCE. 
103 
Elihu Thompson* and J. Trowbridge.*)* The former employed 
powerful alternate-current dynamos and high-frequency ap¬ 
paratus. The latter, during the year just closed, has com¬ 
pleted the installation of a plant of 20,000 storage cells in the 
Jefferson physical laboratory of Harvard College. This 
enormous battery represents a potential of over 40,000 volts. 
During 1889 10,000 cells were used in conjunction with high- 
tension transformers in the study of explosive effects of elec¬ 
trical discharges. Sparks nearly 7 feet in length were then 
produced. Deferring to the electrostatic and electro-magnetic 
action within the vicinity of such sparks, Professor Trow¬ 
bridge remarks: 
“ The electrostatic action does not extend to the great distances readied 
by the electromagnetic effect of the field. While the latter can be de¬ 
tected many miles, the electrostatic effect is confined to a few feet. Thus 
a spark of 6 or 7 feet in length (180 to 210 centims.) is vastly inferior to 
one of 2 or 3 inches (5 to 7.5 centims.) for the purposes of wireless 
telegraphy.” 
This great battery has been used also with striking ad¬ 
vantage in the production of X-ray effects. 
Only the most meager details of the recent attainments in 
wireless telegraphy have yet been published, but it seems the 
Branly coherer in one or another form is still the essential 
device in the receiving apparatus. 
Closely associated with the recent development of the in¬ 
duction coil for X-ray work has been the introduction of the 
electrolytic interrupter. The principle involved is really an 
old one, but the application is recent. 
Terrestrial Gravitation .—Physicists have long sought more 
or less in vain for some static method of measuring satis¬ 
factorily the force of gravitation. The dynamic method by 
pendulums is completely debarred from use on shipboard, 
for example, and in the absence of other equally exact 
methods the intensity of gravity over vast areas of the sur- 
* Scientific American Supplement, No. 927. 
t Phil. Mag., Lond., Edin., and Dub. (5), July, 1900; also (5), Sept., 
1899. 
