When the resistance of x is greater than that of R1, as in an insulation test, the resistance in r is made less than that in R, in order that r and R may have such a proportion one to the other as will enable the coils in R1 to balance a resistance in x, greater than their own, that is to say, greater than 11,100 ohms; thus r : R :: R1 : x, or 10 : 1000 :: 10,000 : 1,000,000, the resistance in the line to be tested would be 1,000,000 ohms, supposing the values of r, R and R1 to be respectively 10, 1000, and 10,000 ohms.
When the resistance to be tested is less than that of the least coil in R1 (1 ohm), then the resistance in r is made greater than in R. Thus r : R :: R1 : x, or 100 : 10 :: 2 : 0·2; the resistance of the line to be tested would in this case be 1/20 of an ohm.
Manipulation.—In all cases the key in connection with the battery should first be depressed, then the galvanometer key, making very short contacts by the latter, just sufficient to show the direction of the deflection, until the coils in R1 are nearly adjusted, otherwise considerable time will be lost in making a series of tests, owing to the swing given to the needle, which will take some little time before it again remains steady at zero. When once the coils in R1 are adjusted, and a balance obtained, it should be ascertained whether the needle will remain steady when contact is made and broken.
Test Tables.—In connection with a system of testing electrical submarine mines, for the sake of convenience and simplicity it is necessary to use a table (termed a "Test Table"), on which all the apparatus used for the purpose of testing are fixed. Several forms of tables have been designed for such a purpose. At [Fig. 95] is shown the method of arranging such a table.[L]
A is an astatic galvanometer placed between two switch plates, B and C; ten other similar switch plates, 1, 2, 3, 4, D, 5, 6, 7, E, and 8, are arranged in front of the galvanometer A; F, G, and H are three terminal plates; K is a box of resistance coils used in connection with the thermo galvanometer M; L is a firing key, and N a battery commutator; O is a three-coil galvanometer; R is a Wheatstone balance (Post-office pattern).
The ten switch plates, 1, 2, 3, 4, D, &c., are used for the connection of any particular line to be tested, as well as for the earth connections and instruments employed in that operation.
"Sea Cell" Tests.—The arrangement shown in the figure is that required in connection with the sea cell test, and Mr. Brown's method of keeping certain earth plates in a bucket instead of in the sea.