John Roche

The radius astronomicus in England

Summary This survey traces the history of the astronomers cross staff on the Continent from Levi ben Gerson to Gemma Frisius, in England from John Dee to John Greaves, and again on the Continent from Tycho Brahe to Adrian Metius. The emphasis throughout is on sources and influences, on distinguishing the various kinds of cross staff, and on clarifying terminology.

Axial vectors, skew-symmetric tensors and the nature of the magnetic field

The direction assigned to the magnetic field today derives historically from magnetic navigation. The author argues that this is a null direction and that the true action of the magnetic field occurs in a plane perpendicular to the conventional direction. This has been recognized in physics since Ampere, but the momentum of tradition and the mathematical convenience of established conventions and notation appears to have prevented its widespread acceptance.

Harriot's ‘Regiment of the Sun’ and its Background in Sixteenth-Century Navigation

It is well known that Richard Hakluyt in a publication of 1581 congratulated Sir Walter Ralegh for employing Harriot to teach him and his many sea captains the sciences of navigation. Even more important, however, was the navigational research carried out by Harriot on behalf of Ralegh. He made important theoretical advances in map theory and in navigational astronomy, carried out the astronomical observations needed for a reform in navigational tables, and designed and himself tested at sea improved navigational instruments. Harriot had many other responsibilities in connexion with Raleghs enterprises. From August 1585 to June 1586 Harriot was in Virginia, and in 1589 he was listed as one of Raleghs colonists in Munster. He collected intelligence concerning America for Ralegh, and his publication of 1588 was effective as propaganda for Virginia. Harriot was also entrusted with financial, and even political responsibilities by Ralegh. Instructing Ralegh and his captains in navigation was an important part of Harriots work but it is more likely that he did this as the occasion demanded, rather than on a regular basis.

Introducing motion in a circle

Motion in a circle troubled Newton and his contemporaries and troubles students today. This article attempts to give a clear presentation of certain aspects, particularly centripetal acceleration and centrifugal force.

Reply to Comment `On Maxwell's displacement current'

This brief note replies to two comments made by Professor Oleg Jefimenko on my recent article (March 1998) on Maxwells displacement current.

Introducing electric fields

The clear introduction of basic concepts and definitions is crucial for teaching any topic in physics. I have always found it difficult to teach fields. While searching for better explanations I hit on an approach of reading foundational texts and electromagnetic textbooks in ten year lots, ranging from 1840 to the present. By combining this with modern techniques of textual interpretation I attempt to clarify three introductory concepts: how the field is defined; the principle of superposition and the role of the electrostatic field in a circuit.

Introducing simple harmonic motion

Simple harmonic motion is an important topic for physics students and has wide applications in the world around us. A clear introduction to the subject is therefore vital. This article stresses the need for a good description of the phenomenon and uses the device of the auxiliary circle to help show the key relationships between acceleration, displacement, time, velocity and phase.