什么是避孕
人类生理学术语,指有意防止怀孕的措施。最常见的方法是绝育术,手术简单,但有效率超过99%。许多避孕方法会损害健康;最安全的方法包括使用阻隔式避孕器以及危险期推算法。避孕药是利用雌激素或孕酮来抑制卵巢排卵。“事後丸”是含有大剂量的雌激素和孕激素或仅有後者的药片,是在性交後避孕的方法之一。口服避孕药最严重的副作用就是可能会造成血栓疾病。子宫内避孕器(IUD)是植入子宫内的塑胶或金属器械,引起子宫内膜发生轻度炎症,从而抑制了排卵,防止了受精,或防止了受精卵着床。1970年代和1980年代,有的避孕器被禁止贩卖,因为发现它们的副作用是高危险性的骨盆炎症、异位妊娠和自发性败血性流产等疾病。保险套、阴道海绵条、阴道隔膜和子宫帽等阻隔式避孕器,以阻止精入进入子宫。保险套还可以预防性传染病(STD)的流传。杀精剂可协助防止精子越过这些阻隔,因此可提高这些器械的效力使之接近100%。危险期推算法是估算排卵期,这样就可在这每月中妇女最易受孕的大约六天内避免性交。除记录个人月经周期外,还可每日测体温,因为基础体温升高时就意味排卵。这两种方法还常再结合第三种方法,即观察子宫颈黏液的变化,这可预示易孕时期。正在实验中且使用较少的节育法还有男性口服避孕药。
contraception
Birth control by prevention of conception or impregnation. The most common method is sterilization. The most effective temporary methods are nearly 99% effective if used consistently and correctly. Many methods carry health risks; barrier devices and avoidance of intercourse during the most fertile period are safest. Hormonal contraceptives use estrogen and/or progesterone to inhibit ovulation. The “morning-after pill” (high-dose hormones) is effective even after intercourse. The most serious side effect of oral contraceptives is the risk of blood-clotting disorders. Intrauterine devices (IUDs) are placed inside the uterus and appear to cause a mild endometrial inflammation that either inhibits fertilization or prevent a fertilized egg from implanting. Certain types were taken off the market in the 1970s and '80s when it was found that their side effects included a high incidence of pelvic inflammatory disease, ectopic pregnancies, and spontaneous septic abortions. Barrier devices, such as condoms, diaphragms, cervical caps, female condoms (vaginal pouches), and vaginal sponges, prevent sperm from entering the uterus. Condoms also prevent sexually transmitted diseases. Used with spermicides, condoms are nearly 100% effective. Fertility awareness techniques have evolved from keeping track of the menstrual cycle (the so-called “rhythm method”; see menstruation) to avoid intercourse around the time of ovulation; tracking body temperature and cervical mucus consistency can raise effectiveness to more than 80%. Experimental forms of birth control include an oral contraceptive for men.
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什么是孔径测量
对于孔的直径的测量,有直接测量、间接测量和综合测量等测量方法。孔径测量是长度计量技术的主要内容之一。直接测量利用两点或三点定位,直接测量出孔径的方法,也是最常用的孔径测量方法。根据被测孔径的精度等级、尺寸和数量大小,可以采用能测孔径的通用长度测量工...
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什么是阿非利堪人
旧称布尔人(Boer)操阿非利堪斯语,且具荷兰或法国新教徒血统之南非人。阿非利堪人原称布尔人(意为农夫),特别是从开普殖民地(CopeColony)移居特兰斯瓦(Transvaal)及橘自由邦(OrangeFreeState)的早期居民。他们是虔诚的喀尔文教徒(Calvinist),自视为上帝的选民,天命他们来统治这...
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什么是清朝
Qingdynasty,亦拼作Ch'ingdynasty,或称为Manchudynasty。中国最後一个帝制皇朝。西元1636年满族首度以“清”为国号在满洲建立王朝,而後当满洲人统治中国时,继续沿用这个国号。清朝期间,中国的领土大幅扩张,人口也大为提高。文化态度则极为保守,新儒学仍是居於主导地...
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什么是似稳电磁场
交变电流的频率较低时,在电流附近,与之相距比该频率的电磁波在真空中的波长少得多的区域内,可以忽略电磁场的推迟效应,这样的区域中的电磁场即为似稳场或准稳场。根据电磁场理论,变化的磁场要产生电场,变化的电场要产生磁场。前者是电磁感应的作用,后者是位移电流的作...