RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
39 
conductor and not upon its shape, conditionally that the conductor passes through the 
coils carrying the current. Thus the difference of potential at the extremities of a 
conductor ACDB is not altered if its shape is changed to AEFB. The segments 
which form the ends of the conductors are insulated from each other and from the 
discs. 
The brushes consist of phosphor-bronze wires stretched by two spiral springs, and 
resemble violin bows. Each brush makes contact with one or two segments over a 
length varying from 5 to 6 cm., and leaves a segment at a tangent thus making the 
pressure greatest at the mid-point of contact. Petrol is employed as a lubricant. 
There are two principal ways of using the apparatus. In the first the ten brushes 
are included in a circuit so as to be in series. When each brush is in contact with a 
single segment, the differences of potential due to five rotating conductors are added 
together, the remaining five conductors being ineffective. When each brush connects 
two neighbouring segments the ten rotating conductors are arranged in five sets of 
two in parallel and the total potential difference is the same as before. It is easily 
seen that by having a comparatively large arc of contact between each brush and a 
segment (or segments) and twice as many segments as brushes, the circuit made 
through the brush contacts is never broken. 
In the second method, the brushes are divided into two sets of five in parallel and 
the total potential difference is the same as that of a single rotating conductor. 
All the coils are wound with bare copper wire on hollow marble cylinders having 
double-threaded screw grooves cut on the surfaces. The two wires on any one 
cylinder form two adjacent helices which may be connected in series or in parallel. 
In the general use of the instrument they were connected in series, but they may at 
any time be disconnected from each other and an insulation test made between them. 
There are eight helices in all and these are connected by means of small concentric 
cables to a plug board and commutators which enable the direction of the current in 
any coil to be changed at will. 
Each of the cylinders is mounted on a strong metal support and its position with 
regard to a disc may be altered with ease by screw adjustments. The distance 
between the mid-planes of two coils is measured by means of microscopes. 
The electric motor used for driving is situated at a considerable distance from any 
one of the coils, and its influence on the result was calculated and also experimentally 
proved to be negligibly small. A commutator is fixed to the axle of the motor, and 
this serves to charge and discharge a condenser placed in one arm of a Wheatstone 
bridge. By keeping the bridge permanently balanced, the speed of the Lorenz 
apparatus is maintained constant. A directly driven chronograph enables the speed 
to be calculated. 
The whole instrument is supported on gun-metal rails embedded in a solid block 
made with Keene’s cement, sand, and ballast. With the exception of the motor, no 
magnetic material is used in the construction of the apparatus or supports. The 
