Measurements of the intensity of a magnetic field by the Lenard Bismuth Spiral are due to the alteration of resistance which this metal exhibits when placed in a magnetic field.  A thin insulated wire of chemically pure bismuth is bifilar wound in a flat spiral form, the ends of this winding are soldered to flat copper strips which are held together in an ebonite handle and fitted with terminals.  The spiral is shielded from damage by mica discs cemented on, it is only about 1 mm thick and can therefore be introduced into very narrow fields, for instance between the armature and poles of a dynamo.  The alteration in resistance varies with the lines of force in the field under test, a 5% variation in resistance corresponding practically to about 1000 lines of force; to ensure greater accuracy a calibration curve is supplied with each instrument.  The readings vary with the temperature and, in accurate work, the instrument should be used at the temperature of calibration, the coil must therefore not be heated by the current which should be kept as low as possible (for No. 504 at 15 milliamp. and for No. 504a at 8 milli-amp. max) and closed only for a very short time.