In my laboratory we make great use of the method of tracing lines of force described in Glazebrook and Shaw's Practical Physics. The field is modified in various ways; for instance, the fixed magnets are sometimes placed so as to somewhat resemble in their disposition the field magnets of a dynamo: sometimes a circular piece of soft iron is placed in the field, and the effect of its induced magnetization examined. The students like these exercises, and the results (of which some are exhibited) are very beautiful. We use steel bars about 8 cm. long for fixed magnets, and small rather heavy lozenge-shaped needles, about one to two cm. long, as what I shall call pointers.