1 8 BULLETIN OF THE LABORATORIES 
justment of the instrument. If the resistances of the various pieces of 
electrical apparatus in the laboratory are measured, a moment’s calcula- 
tion will show just what arrangement will give the greatest amount of 
current, or any amount desired, with a certainty and precision that is very 
satisfactory. This increased precision is possible with so little trouble 
by the use of this cell-connecter that it is well worth one’s while to 
gain it. 
Larger batteries can be managed either with a similar instrument 
having a greater number of plates and connecting plugs, or by attach- 
ing to each pair of plates in the form described, a battery of cells, ar- 
ranged in series or parallel, according as high potentials or heaVy cur- 
rents are most required. 
Another form of the instrument has been tried. It is more easily 
made, but is less convenient in use. It consists simply of two rows 
of binding posts on a base. These are connected by short wires in 
various ways, for various combinations Two rows of metal contact 
points, each carrying a sliding switch which could be placed upon the 
next point of either the same or the other row, would also be a conve- 
nient form. By this method, however, there would be difficulty in 
securing good contact between points and switches. 
This instrument is especially useful for a complete experimental 
proof of Ohm’s law. A student with one of them, an ammeter and 
voltmeter, or in place of the last two, a tangent galvanometer having 
both low and high resistance coils, can verify the law for a large num- 
ber of representative combinations in a wonderfully short time. 
It has been found a great convenience in this laboratory to have 
one of them upon the demonstrator’s desk, with a dozen open-circuit 
cells attached. These require no attention for six months at a time, 
are always ready for use and do not polarize seriously in the short time 
necessary for an ordinary lecture experiment. The ^‘old reliable” of 
the laboratory, a ten-cell plunge battery, has been thrown out of em- 
ployment since the advent of the cell-connecter. It is surprising how 
much can be done by the open circuit cells, with the aid of their pow- 
erful auxiliary. 
It is hoped that it may prove as great a convenience in other lab- 
oratories as it has in this. Evidently it can be used with secondary 
batteries as well as primary. In this way it gives perfect control of 
very heavy currents. 
