. Practical wireless telegraphy; a complete text book for students of radio communication . nt-detecting instrument kno^the galvanometer. This instrument maytake one of several forms, but the typeshown in Fig. 46 is the least difficult tounderstand. A rectangular coil of sev-eral turns of copper wire D is suspendedbetween the poles of a horse shoepermanent- magnet, P, P. Between thepoles is the stationary iron core C* Thecoil is suspended from the screw F and thecurrent to be measured enters at the wireA and leaves by the wire B. Whencurrent is passed through the coil B, ittends to turn so as
. Practical wireless telegraphy; a complete text book for students of radio communication . nt-detecting instrument kno^the galvanometer. This instrument maytake one of several forms, but the typeshown in Fig. 46 is the least difficult tounderstand. A rectangular coil of sev-eral turns of copper wire D is suspendedbetween the poles of a horse shoepermanent- magnet, P, P. Between thepoles is the stationary iron core C* Thecoil is suspended from the screw F and thecurrent to be measured enters at the wireA and leaves by the wire B. Whencurrent is passed through the coil B, ittends to turn so as to include the greatestnumber of lines of force but is resisted bythe torsion of the suspending wires. If apointer and suitable scale are attached tothis coil, comparative readings of th?strength of current may be made. In-struments of this construction are sen-sitive and will easily measure a currentof .000001 of an ampere. It is an impor-tant instrument to demonstrate the ele-mentary principles of electromagneticinduction, and should be a part of allstudents equipment. Now if the coil. Fig. 46—Simple Galvanometer. D had several thousand ohms resistance, the galvanometer might be calibrated in volts and em-ployed as a voltmeter. If, on the other hand, coil D were wound with a few turnsof relatively coarse wire, it might be calibrated in amperes and would, therefore,be known as an ammeter. As an ammeter it would be connected in series with thecircuit under measurement. •The Core C intensifies the field across the air gap P to P. ELECTROMAGNETIC INDUCTION. 43 (o) The voltmeter may be constructed along the lines of galvanometer. Asimple, drawing of the Weston voltmeter appears in Fig. 47. A rectangular coil of fine wire, A, B,is mounted on the metal bobbin G. It issupported by jewelled bearings and heldin the zero position of the scale by thespiral springs, S-1 and S-2, throughwhich the circuit of the coil is com-pleted. When the pointer is in zero po-sition, the coil rests a
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Keywords: ., bookcentury1900, bookdecade1910, booksubjecttelegra, bookyear1917
