GYROSCOPIC COMPASS METHODS OF SURVEYING 213 



The gyroscopic compass constructed to meet these 

 demands is shown in Fig. 2 (Plate XIII) and set in the 

 lowest part of the apparatus (Fig. 1). A ring-shaped vessel 

 a filled with mercury is fixed on the rotatable measuring 

 case in a housing with the aid of bows b. A ring-shaped 

 float c in the mercury vessel holds the wheel cap e by a neck 

 d. In this cap or case runs the gyrowheel on ball bearings, 

 the wheel itself being of nickel steel and having a short- 

 circuit rotor pressed on it. The stator of the small alter- 

 nating-current motor which drives the wheel is fixed in the 

 wheel case, and it is supplied with a 400-cycle per second 

 current, by means of fine silver wiring, causing the wheel 

 to make 25,000 r.p.m. 



The construction of such a quick running alternating- 

 current motor with short-circuit armature is extremely 

 skillful; the high number of revolutions demands much 

 copper in the rotor so that the turning moment be small, 

 otherwise the wheel will not exceed a definite speed range. 



The wheel hangs in its case as deep as possible below 

 the float compatible with the tube width against which it 

 would bump if very deep. In this position gravitation 

 tends to hold the axis horizontal and the axis adjusts itself 

 to the meridian by Foucault's principle, stated above, 

 because the entire floating system is arranged rotatable 

 about the center rod /. A directing force of some tenths 

 gram-centimeter suffices to turn this small gyrocompass 

 but not the whole measuring box, so for that reason the 

 following arrangement is adopted. On the floating system 

 is fixed a contact bead g which, when the float with the wheel 

 turns right or left, makes contact with a contact spring 

 on right or left and in this way a so-called ''turning, 

 take-up, or compensating motor" changes its rotational 

 sense. This is to be found in the uppermost part of the 

 inclination measurer (Fig. 3). It is a small direct-current 

 motor with double armature winding and commutators 

 on both sides; and by the contact bead one or the other 

 of the windings is cut out causing the armature to rotate 

 in the opposite way. 



