stant deflection of approximately 110 tangent divisions is obtained on its scale. The constant having been thus obtained and noted, the galvanometer is next connected as shown in Fig. 93, such that the current from the bat tery E will tend, by flowing through both windings, to produce a deflection in opposite directions of the galvanometer needle. It is obvious, then, that if the insulation resistance of the line be infinite, and there be no leakage, the currents through r and rl, and consequently through the two sides of the galvanometer winding, will be equal and opposite, and, as a natural consequence, no deflection will result; if, on the other hand, there be a leakage, either local or general, at any point on either side of the lines, the current through r will be greater than that through rl, and a deflection will result. By means of a series of specially prepared tables, this deflection can be rendered in terms of the insulation resistance of the line, in "megohms per mile." The conductor resistance of the loop naturally enters into these calculations, and is duly provided for in the tables alluded to. In the event of a single line requiring a test where no looping can be effected, the distant end is put to earth through a combined resistance of 10,000 ohms, and the winding of the galvanometer normally connected to rl, and the deflection due to the line, as calculated for perfect insulation, is deducted from the observed deflection, then the remainder, multiplied by 2, will give the required deflection from which the insulation resistance can be deduced, as before, by reference to the tables. INSTRUMENTS, BELLS, &c. Electric Bells and Alarms By F. E. PoWELL. 8. d. 07 ... 0 7 Electric Batteries; How to make and use them. Something About X Rays for Everybody. By E. 1 REVERT. The Intensity Coil and the Crookes Tube; Experiments with X Rays; The Fluorscope and other Apparatus; Index. Illustrated. 1s, and Cloth ... ... Practical Radiography. By H. S. Ward. ... ... ... 1 0 20 2 6 Portative Elictricity. By J. T. NIBLETT. Illustrated 26 Radiography: Its Theory, Practice, add Applications. the Construction and Working of Shock, Medical and Spark Coils. By 3 0 3 0 Practical Bookf or Prctical Men. 13 pp. and 177 illustrations Electric Bell Fitting. A Practical Treatise on the fitting up and maintenance of Electric Bells and all necessary apparatus. By F. C. ALLSOP. Nearly 150 Illustrations *Electrical Instrument Making. For Amateurs. A Practical Handbook by S. R. BorTONE. With 96 Illustrations. 3 8 Electric Toys. Electric Toy-Making, Dynamo Building, and Electric Motor Construction for Amateurs. By T. O'CONNOR SLOANE, Ph.D. The Bellhanger's Handbook, F. B, Badt.... A. T. HARE ... ... ... 60 Those marke 1 are recommended by the Examiners of the City and Guilds of London Institute, |