describe the working principle of synchroscope and phase sequence indicator
Synchroscope A polarized-vane synchroscope has a field winding with a phase-shifting network arranged to produce a rotating magnetic field. The field windings are connected to the "incoming" machine. A single-phase polarizing winding is connected to the "running" system. It is mounted perpendicular to the field winding and produces a magnetic flux that passes through the moving vanes. The moving vanes turn a shaft that carries a pointer moving over a scale. If the frequency of the source connected to the polarizing winding is different from the source connected to the field winding, the pointer rotates continually at a speed proportional to the difference in system frequencies (the beat frequency). The scale is marked to show the direction of rotation corresponding to the "incoming" machine running faster than the "running" system. When the frequencies match, the moving vanes will rotate to a position corresponding to the phase difference between the two sources. The incoming machine can then be adjusted in speed so that the two systems are in phase agreement.
Tape measure On December 6, 1864 patent #45,372 was issued to William H. Bangs of West Meriden, Connecticut. Bang's rule was the first attempt in the United States to make a spring return pocket tape measure.  The tape could be stopped at any point and held by the mechanism. The tape could be returned to the case by sliding a button on the side of the case which then allowed the spring to pull the tape back into the case. [2]