Context and question: The Warnerke Standard Sensitometer consisted of a frame holding an opaque screen with an array of typically 25 numbered, gradually pigmented squares brought into contact with the photographic plate during a timed test exposure under a phosphorescent tablet excited before by the light of a burning Magnesium ribbon. The speed of the emulsion was then expressed in 'degrees' Warnerke (sometimes seen as Warn. or W.) corresponding with the last number visible on the exposed plate after development and fixation. Each number represented an increase of 1/3 in speed, typical plate speeds were between 10 and 25 Warnerke at the time.
What was the range of the number of squares in a Sensitometer?
Answer: unanswerable
Context and question: The rotating armature consists of one or more coils of wire wound around a laminated, magnetically "soft" ferromagnetic core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet (an electromagnet). The magnetic field produced by the armature interacts with a stationary magnetic field produced by either PMs or another winding a field coil, as part of the motor frame. The force between the two magnetic fields tends to rotate the motor shaft. The commutator switches power to the coils as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops (like a compass needle does), but rather keeps rotating as long as power is applied.
What rotates the motor shaft?
Answer: force between the two magnetic fields
Context and question: The Quran and Muhammad talked about racial equality and justice as in The Farewell Sermon. Tribal and nationalistic differences were discouraged. But after Muhammad's passing, the old tribal differences between the Arabs started to resurface. Following the RomanPersian Wars and the ByzantineSassanid Wars, deep rooted differences between Iraq, formally under the Persian Sassanid Empire, and Syria, formally under the Byzantine Empire, also existed. Each wanted the capital of the newly established Islamic State to be in their area. Previously, the second caliph Umar was very firm on the governors and his spies kept an eye on them. If he felt that a governor or a commander was becoming attracted to wealth, he had him removed from his position.
Who was the second caliph?
Answer:
Umar