ye 
+ 
eee 
_ vanometer, for no reason can 
872 J. Trowbridge—Induced currents and derived circuits. 
Art. XLUL—Contributions from the Physical Laboratory of 
Harvard College—No. IV. Induced currents and derived 
ewrcuits ; by JOHN TROWBRIDGE. 
THE expression for the intensity of an induced current, de- 
duced by Neumann and Sir William Thomson, is as follows: 
a , in which & is a coefficient depending upon the re- 
k dt 
sistance of the complete wire in the secondary circuit, and U is 
a certain “force function” which depends solely upon the form 
and position of the wire at any instant, and on the magnetism 
of the influencing body. The expression, in general language, 
is as follows: 
‘When a current is induced in a closed wire by a magnet In 
relative motion, the intensity of the current produced is propor- 
tional to the actual rate of variation of the “ force function” by 
the differential coefficients of which the mutual action between 
the magnet and the wire would be represented if the intensity 
of the current in the wire were unity.” 
This investigation was undertaken to ascertain if the laws of 
derived circuits apply to the currents of induction, which are 
represented by equations of which the above is a type. re- 
ecting galvanometer of large resistance was included in the 
secondary circuit, and connected by copper wires of very small 
resistance with the coil in which the secondary currents were 
produced: the resistance of these wires was infinitessimal in 
comparison with the resistance of the galvanometer. e gal- 
vanometer was then shunted. The first two columns of the 
following table show that, with an inappreciable resistance out- 
side of the galvanometer coils, the shunts made no difference 10 
the deflection of the galvanometer needle when the shunts were 
not less than three ohms. Below this the current divided. 
The resistance of the galvanometer was 5880 ohms, and the last 
numbers in the second and third columns show that an equal 
Exterior Shunts, Exterior 
Resistances,} in ohms. Defiections. | Resistances, | Defiections. 
in obms. ‘in ohms, 
0 3 210 10 210 
. 4 210 20 210 
- 5 210 30 210 
. 6 210 a0 210 
es 5880 210 100 190 
impulse was transmitted through both the shunt and the gal- 
Ss assigned why it should take 
