382 
MR. E. H. GRIFFITHS OH THE VALUE OF 
All connecting wires of any length were carried overhead, and hung by silk threads 
from a net suspended about 7 feet above the table. Thus all were visible, and could 
be traced without difficulty. All keys and instruments through which communication 
with earth could be made were placed on slabs of paraffin or ebonite, and the accumu¬ 
lators were also insulated. Each lead was placed as close as possible to its return to 
avoid any electro-magnetic action. The resistance of the galvanometer (7^ was about 
9000 ohms. Instruments of such high resistance speedily indicate any leakage in 
the circuit, and it was not until we had given considerable time and attention to the 
insulation that we were free from trouble on this account. 
Fig. 2. 
The galvanometer (an astatic one manufactured by the Scientific Instrument 
Company, and the field so adjusted that the time of a single oscillation was about 
4 seconds) showed no deflection unless connected with the Clark cells, although, at 
times, the difference of potential between the screws 2 and 4 amounted to 10 volts. 
If the fingers were placed on the ends of the wires communicating with it, the spot 
of light could be thrown nearly off’ the scale. The galvanometer was placed at the 
farther end of a tunnel (4 feet in length) of which the nearer end was closed by a 
sheet of ground glass. The beam of light was thrown in thi'ough the side of the 
tunnel, reflected on to the galvanometer mirror and thence on to the ground glass, which 
was protected by a hood from other illumination. Thus the “ spot ” was easily visible, 
