From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Page discussing demagnetising fields, magnetic induction, and the effect of air gaps in permanent magnets and magnetos.
| Identifier | ExFiles\Box 35\1\ scan 178 | |
| Date | 11th August 1920 | |
| -8- Contd. of this demagnetising field is proportional to the width of the gap for small gaps. Even if we suppose the magnetisation of the ring to be rigid i.e. unaltered by changing magnetic influences (this is of course, what we aim at in making a permanent magnet), nevertheless the increase in the width of the gap from nothing upwards, reduces the induction across the gap, and everywhere in the material, for it must be remembered that useful induction not only consists of magnetisation in the material, but also of induction in the occupied space, which in this case is opposite to the magnetisation. Or we may put it this way, that an air gap causes leakage of the induction from point to point of the ring, which leakage is for small gaps, proportional to the width of the gap. The case of an ordinary magneto magnet and armature is similar. Suppose in the first instance the magnet has a rigid magnetisation, consisting of a flux of lines N. It tends to produce N lines of induction across its armature, but it will only do this if its armature is a perfect keeper of no reluctance which fits between the poles in such a way as to admit of no free polarity. The existence of free polarity due to air gaps causes demagnetising force all over the magnet. This is harmful in two ways:- Contd. | ||