Altitude sundials do not require knowledge of the N-S direction to quantify time in terms of either seasonal or equal hours. Methods for determining the corresponding dial patterns are reviewed, and accurately computed data for a latitude of 51°N presented as horizontal, vertical, rectangular, and pillar dials on both timekeeping systems.

Canada balsam is secreted by cells in the bark of the balsam fir Abies balsamea, a conifer widely distributed across Canada and northern U.S.A. Collected by an extraordinarily labour-intensive process, it was initially exported to Europe for use in pharmacy. However, it found a much wider application after Brewster showed that the refractive index of 1·54–1·55 characterizing the vitreous mass remaining after heating the natural product exceeded that of most resins, and therefore closely matched the indices of ‘crown’ and ‘light (...) flint’ optical glasses. ‘Cooked’ Canada balsam was therefore very suitable for cementing achromatic lenses for telescopes and microscopes, as well as for mounting specimens for microscopical examination. Canada balsam remained pre-eminent for over a century, but the rise of synthetic adhesives and other substitutes has now relegated the natural balsam to a few specialized niches. (shrink)

Classic voltaic batteries of the silver/zinc and copper/zinc types are the ancestors of today's primary cells, and facilitated the development of many aspects of electrical technology. Nevertheless, they appear never to have been studied and evaluated in a quantitative manner, with results recorded in terms of volts, amps, ohms, and watts. Research of this nature is reported here, and has been conducted for the most part with copper/zinc cells. Log–log graphs of voltage versus load and current, and power versus load, (...) are presented for many electrolyte systems. It has been shown that, although the textbook electrolyte of dilute sulphuric acid does work, it is an order of magnitude inferior to a solution containing some additional nitric acid. The latter diminishes the current‐limiting phenomenon of polarization, and was in fact used by Davy, Faraday, and other early investigators. A quantitative consideration of Nicholson and Carlisle's discovery of the electrolysis of water with a silver/zinc voltaic pile is followed by examination of the electrolysis of pure water, trough batteries, and Davy's isolation of potassium and sodium. Every battery gives maximum power when its resistance is adjusted to match the resistance of the load: the maximum output of the ‘Great Battery’ of the Royal Institution is assessed at no more than 3 kW. The paper concludes with a note on the recognized hazard of long‐term exposure to mercury vapour and its possible relevance to the health of Michael Faraday. (shrink)

This paper examines the various types of small, portable heliostat which maintain a beam of sunlight in a constant direction. Used mainly in the second half of the nineteenth century, these instruments were intended for optical demonstrations and the illumination of microscopes and similar apparatus, rather than for astronomical applications. Three basic patterns are distinguished: polar heliostats , biaxial single-mirror heliostats, and coelostats. The theory and geometry of each type is explained, followed by a consideration in chronological order of the (...) corresponding working instruments. Heliostats by Fahrenheit, Meyerstein, 's Gravesande, Gambey, Silbermann, Foucault, Stoney and others are illustrated, and extensive references given. (shrink)

The true form of the seasonal-hour sundial, in both vertical and horizontal planes, has been calculated by spherical trigonometry and displayed with the aid of computer graphics. These grids are presented, and show that the hour lines are in fact shallow curves at sites not on the equator. The curvature becomes very apparent at latitudes exceeding 50°. The true seasonal-hour pattern for a vertical dial at a latitude of 52·6°N is compared with the equiangular scratch dial.

In the early 1800s galvanometers could be constructed with the fine gauges of silk-covered copper or silver wires produced for decorative purposes, but when Faraday was making his classic electrical experiments in 1831 he needed a sturdier gauge of copper wire. Bare copper wire was available in many diameters for mechanical applications, but coils for electromagnetic investigations had to be insulated with string and calico. It was soon realized that the cotton-covered springy iron wire then used to hold out the (...) brims of ladies' bonnets showed how copper wire might be similarly wrapped to provide a flexible insulation. The simple manual machines used by the bonnet-wire makers were readily adapted and improved, and a six-head version was built by William Henley. This craftsman's vision of the growing importance of insulated copper wire was abundantly justified, and he built up a large—but poorly organised—empire in the wire and cable trade. Henley's original multiple-head wrapping machine has been located in the Science Museum, London, and the associated silk-covered copper wire subjected to physical, chemical, and electrical testing. For comparison, the electrical conductivity of the ‘mechanical grade’ copper wire used by Faraday has also been determined. (shrink)

The lodestone is an extremely rare form of the mineral magnetite that occurs naturally as a permanent magnet. It therefore attracts metallic iron as well as fragments of ordinary ‘inert’ magnetite. This ‘magic’ property was known to many ancient cultures, and a powerful lodestone has always commanded a high price. By the eleventh century AD the Chinese had discovered that a freely suspended elongated lodestone would tend to set with its long axis approximately north–south, and utilized this property in the (...) magnetic compass. They also appear to have discovered that this invaluable characteristic could be handed‐on to a steel needle if the latter were contacted with, or stroked by, a lodestone.The magnetism of the lodestone was scientifically investigated by William Gilbert in the sixteenth century, when he defined its ‘poles' and the well‐known rule that ‘like poles repel, unlike attract’. He also studied ‘inclination’ and ‘variation’, and means to aid the preservation of magnetic power. How to concentrate it by ‘arming’ the lodestone with caps or pole‐pieces of soft iron was discovered in the same century. These methods have been repeated, confirmed, and improved. The lodestone occupies a vital place in the history of magnetism, but little beyond Gilbert's work can be reached by historical studies because vastly improved steel or alloy permanent magnets, and electromagnets, replaced it before quantitative measurements were developed. These techniques have therefore been applied retrospectively to both museum specimens and contemporary natural lodestones. A good source of the latter was found to be the igneous complex known as Magnet Cove, Arkansas, and this material has been used as the ‘type example’. All specimens were discrete, well‐rounded, rusty brown pebbles found near the surface. Their unweathered interiors were black titanomagnetite. No significant trace element or crystallographic differences could be found between the lodestones and the magnetically inert material that always accompanied them. The magnetic moment per unit volume of the ‘as‐found’ Magnet Cove lodestones varied between 6.5 and 11.6 emu cm−3, which compares poorly with the hundreds of units characteritzing modern permanent magnets. Hysteresis loops gave a saturation intensity of 27–51 emu cm−3, suggesting that intensity has diminished since formation. This agrees with general experience of magnets, especially in the absence of a ‘keeper’. The initial volume susceptibility of Magnet Cove magnetite was about 0.18 for low fields, and always remained <1. This means that a normal terrestrial magnetic field with a maximum vector <1 Oe is unable to induce even the low magnetic moments we see today, while a field approaching 1000 Oe is required for saturation. These parameters, and the rare occurrence of lodestones as near‐surface fragments, support suggestions that they are the product of a lightning strike upon an exposure of a suitable magnetite. Transient currents averaging 30 000 A have been measured. This would give rise to a zone of potential magnetic saturation at least 12 cm in diameter, to which some of the ejected fragments would be exposed. An attempt to determine the period elapsed since formation of the Magnet Cove lodestone was made by annealing magnetically saturated specimens at temperatures up to 500°C, and measuring J v at weekly intervals for 100 days. The decay curves visually resembled exponential functions, but mathematical tests proved that they were not strictly so. Interpretation was therefore difficult, but a pragmatic procedure involving excessive extrapolation suggested an ‘age’ of about 3500 years. (shrink)