Firing characteristics of a simple coaxial triggeredva cuum gap

The firing and delay characteristics of a simple coaxial type of triggered vacuum gap (TVG) are described and compared with the planar type. The designs are new and differ from those reported earlier. By analogy with gaseous breakdown the statistical and formative time lags have been determined.

Delay time of a vacuum gap triggered by an exploding wire

The experimental results of delay time of a vacuum gap triggered by an exploding wire plasma have been reported. It consists of explosion delay time and propagation delay time. The explosion delay time has been found to be dependent on the parameters of the exploding wire and the exploding wire circuit and is independent of vacuum gap configuration. The propagation delay time depends on the properties of the exploding wire plasma and vacuum gap parameters such as the number of injection slots, gap spacing, gap polarity, etc. In the absence of prebreakdown current in the vacuum gap, the breakdown can be initiated only after the plasma completely bridges the gap spacing. Under this specific condition, it has been shown that the delay time data can be used to calculate the plasma velocity.

Self-excited triggered vacuum gap

The self-triggering action of the TVG under DC conditions using the same power source as the main circuit is described. Because of the low impedance of the power source compared with the thyratron pulse generator, the TVG can be triggered with voltages as low as 85 V. It is found that circuit conditions influence the minimum stable arc currents that can be sustained.

A simple triggered vacuum gap

A simple triggered vacuum gap has previously been described by the authors in this journal (see ibid., vol.5, 415, 1972). Further studies have resulted in improvement of the performance with regard to sensitivity and consistency of the trigger characteristics and immunity from bridging due to metal particles eroded from the arc. The earlier design suffered from rather frequent bridging of the auxiliary gap and showed rather wide scatter in its trigger characteristics. In the present design thermally stable materials like fused quartz, machinable ceramic 'Supramica 500' (Mycalex Corporation of America), lead titanate, barium titanate (LCC HTD) and silicon carbide have been used to insulate the trigger electrode from the cathode. Consistent triggerings free from bridging, at relatively low voltages of 200-400 V have been obtained.

Delay characteristics of a simple triggered vacuum gap

Firing delays of a simple triggered vacuum gap are reported in this paper. The effects of insulating materials in the auxiliary gap, auxiliary gap current, main gap current and electrode separation on the delay have been investigated. The presence of insulating material in the auxiliary gap having low auxiliary gap resistance appears to exhibit large delay. Delay decreases considerably with increase of current in the auxiliary and the main gaps, but it increases with increase of electrode separation. The scatter in the delay is less than 25 ps and 500 ps with supramica (Mycalex Corporation of America) and silicon carbide respectively at lower values of auxiliary gap current and it becomes negligible for supramica at auxiliary gap currents greater than 6A. This investigation appears to indicate that the simple device can be used as a fast switch.

A high gain aluminium electron multiplier

A twenty stage electron multiplier using aluminium as dynode material is described. When operated in DC mode, very stable gains approaching 106 were obtained with input currents of the order of 10-12 A, even after repeated exposures to the atmospheres.

Low-voltage and high-current delay characteristics of a simple triggered vacuum gap

Low-voltage and high-current switching delay characteristics of a simple triggered vacuum gap (TVG) are described using lead zirconate titanate as the dielectric material in the auxiliary gap. This TVG has superior performance at high currents (up to 14 kA was studied) with regard to delay, reliable firing and extended life as compared to the one using either supramica or silicon carbide. The total delay consists of three intervals: to break down the auxiliary gap, to propagate the trigger plasma and to break down the main gap. The data on the influence of the various parameters like the trigger voltage, current, energy and the main circuit energy are given. It has been found that the delay due to the first two intervals is small compared to the third.

Anode Phenomena in Triggered Vacuum Gaps

Arc voltage - current characteristics and threshold current densities occuring during the formation of anode spots in triggered vacuum gaps are reported. Single pulses of 1.65 ms arcing time, which correspond to switching surge currents, are used in the study with copper and aluminum anodes. The threshold values are 1.75 times the values reported earlier using the longer, 8 mis, arcing time. They are found to depend upon the duration of arcing time as well as upon electrode material, surface conditions, electrode size and contact separation. Lateral inhomogenity in the electrode geometry appears to reduce the threshold value by promoting early formation of anode spots.

Middle aged wasps mate through most of the year, without regard to body size, ovarian development and nestmateship: a laboratory study of the primitively eusocial wasp Ropalidia marginata

We studied the mating behaviour of the primi-tively eusocial wasp Ropalidia marginata and the factors that may influence sperm transfer. By introducing a male and a female R. marginata into ventilated transparent plastic boxes, we were able to observe mating behaviour, and it involved mounting and short or long conjugation of the wasps. Dissection of female wasps after the observation indicated that long conjugation is a good behavioural predictor of sperm transfer. This finding makes it possible to obtain mated females without dissecting them every time. We tested the effect of age, season, relatedness, body size and female's ovarian status on mating. Under laboratory conditions, mating success declined rapidly below and above the ages 5-20 days. Within this age range mating success was significantly low in December compared to other months tested. There was no nestmate discrimination, and there was no influence of male and female body size or of the ovarian state of the female on the probability of sperm transfer.

Warfare in stingless bees

Bees are well known for being industrious pollinators. Some species, however, have taken to invading the nests of other colonies to steal food, nest material or the nest site itself. Despite the potential mortality costs due to fighting with an aggressive opponent, the prospects of a large bounty can be worth the risk. In this review, we aim to bring together current knowledge on intercolony fighting with a view to better understand the evolution of warfare in bees and identify avenues for future research. A review of literature reveals that at least 60 species of stingless bees are involved in heterospecific conflicts, either as attacking or victim colonies. The threat of invasion has led to the evolution of architectural, behavioural and morphological adaptations, such as narrow entrance tunnels, mud balls to block the entrance, decoy nests that direct invaders away from the brood chamber, fighting swarms, and soldiers that are skilled at immobilising attackers. Little is known about how victim colonies are selected, but a phylogenetically controlled analysis suggests that the notorious robber bee Lestrimelitta preferentially attacks colonies of species with more concentrated honey. Warfare among bees poses many interesting questions, including why species differ so greatly in their response to attacks and how these alternative strategies of obtaining food or new nest sites have evolved.

Gender, Politics and Democratisation in Cameroon

Gender perceptions, religious belief systems, and political thought have excluded women from politics, for ages, around the world. Combining feminist and modernisation theorists in my theoretical framework, I examine the trends in patriarchal Europe and I highlight the gender-sensitive model of the Nordic countries. Retracing local gender patterns from precolonial to postcolonial eras in sub-Saharan Africa, I explore the links between perceptions, needs, resources, education and women's political participation in Cameroon. Democratisation is supposed to open up political participation, to grant equal opportunities to all adults. One ironic feature of the liberalisation process in Cameroon has been the decrease of women in parliamentarian representation (14% in 1988, 6% in 1992, 5% in 1997 and 10% in 2002). What social, cultural and institutional mechanisms produced this paradoxical outcome, the exclusion of half the population? The gender complementarity of the indigenous context has been lost to male prevalence privileged by education, church, law, employment, economy and politics in the public sphere; most women are marginalised in the private sphere. Nation building and development have failed; ethnicism and individualism are growing. Some hope lies in the growing civil society. From two surveys and 21 focus groups across Cameroon, in 2000 and 2002, some significant results of the processed empirical data reveal low electoral registration (34.5% women and 65.9% men), contrasted by the willingness to run for municipal elections (33.3 % women and 45.2% men). The co-existence of customary and statutory laws, the corrupt political system and fraudulent practices, contribute to the marginalisation of women and men who are interested in politics. A large majority of female respondents consider female politicians more trustworthy and capable than their male counterparts; they even foresee the appointment of a female Prime Minister. The Nordic countries have institutionalised gender equality in their legislation, policies and practices. France has improved women's political inclusion with the parity laws; Rwanda is another model of women's representation, thanks to its post-conflict constitution. From my analysis, Cameroonian institutions, men and more so women, may learn and borrow from these experiences, in order to design and implement a sustainable and gender-balanced democracy. Keywords: democratisation, politics, gender equality, feminism, citizenship, Cameroon, Nordic countries, Finland, France, United Kingdom, quotas, societal social psychology.

Triggered vacuum gap for initiating electric arcs in vacuum

Describes a simple triggered vacuum gap developed for initiating electric arcs in vacuum which uses the property that the voltage required to breakdown a gap in vacuum in the presence of a solid insulating material is considerably less than the voltage required in the absence of such material. In this triggered vacuum gap a solid insulating material is used in the angular space between the main cathode and the concentric trigger electrode forming the auxiliary gap. Different materials like epoxy resin, Teflon (PTFE) and mica have been used. The trigger voltage was found to vary in the range 560-1840 V. The results with epoxy and Teflon were unsatisfactory because the trigger voltages showed wide scatter and the auxiliary gap was soon bridged by metal particles eroded from the electrodes. Though the trigger voltages required with mica were relatively high, consistent triggering could be obtained for a large number of trials before the auxiliary gap was bridged. This was probably due to better thermal stability of mica as compared with either epoxy or Teflon.

Le Grand Tour Du reste de l’Europe jusqu’à la Laponie – et retour

Rencontre entre le Sud et le Nord Depuis des siècles, les pays méditerranéens ont entretenu des relations actives avec le Nord de l’Europe. Les regions du Nord ont été conquises et converties, manipulées et colonisées, mais aussi intégrées dans les réseaux commerciaux, scientifi ques, culturels et touristiques. Les voyageurs du Sud de l’Europe ont peu à peu découvert les regions plurielles du Nord de la Finlande, de la Suède, de la Norvège et de la presqu’île de Kola. L’attraction focntionnait aussi dans l’autre sens. Pour les gens du Nord, scientifi ques ou artistes, le Sud de l’Europe constituait une destination aussi exotique aue la Laponie pour ceux du Sud. Dans ce recueil, nous proposons dix points de vue sur ces échanges et réseaux entre la Sud et le Nord. Le premier chapitre présente Tornio, connue très tôt comme la porte de la Laponie, les premières relations scientifi ques entre le Nord et le Sud, ainsi que les exhibitions de Samis dans les métropoles européennes. Le deuxième chapitre propose une analyse des experiences et des impressions de voyageurs italiens, espagnols et français. Le dernier chapitre est consacré aux voyages d’artistes et écrivains fi nlandais en France, principalement à Paris. Ces rencontres entre le Sud et le Nord qui participèrent à la remise en cause, la relativisation et la construction des identities nationals et ethniques, permirent aussi de créer une conscience européenne commune.

The Single Langmuir Probe under Orbital-Limited Conditions in a Low-Density Collisional Plasma

The cylindrical Langmuir probe under orbital-limited conditions was used to determine the charge density in a low-density collisional plasma. The Langmuir's theory was applied to both electron and ion saturation currents in their respective accelerating regions. Present study indicates that the length of the probe significantly affects the probe characteristics. A probe of suitable length under orbital-limited conditions may be useful under the experimental conditions where the radius of the probe is much smaller than the Debye lengt.

Parameter estimation in water-distribution systems by least squares

The weighted-least-squares method using sensitivity-analysis technique is proposed for the estimation of parameters in water-distribution systems. The parameters considered are the Hazen-Williams coefficients for the pipes. The objective function used is the sum of the weighted squares of the differences between the computed and the observed values of the variables. The weighted-least-squares method can elegantly handle multiple loading conditions with mixed types of measurements such as heads and consumptions, different sets and number of measurements for each loading condition, and modifications in the network configuration due to inclusion or exclusion of some pipes affected by valve operations in each loading condition. Uncertainty in parameter estimates can also be obtained. The method is applied for the estimation of parameters in a metropolitan urban water-distribution system in India.