A Brief History of Clock
Clocks
At best, historians know that 5,000-6,000 years ago, great civilizations in the Middle East and North Africa started to examine forms of clock-making instead of working with only the monthly and annual calendar. Little is known on exactly how these forms worked or indeed the actual deconstruction of the time, but it has been suggested that the intention was to maximize time available to achieve more as the size of the population grew. Perhaps such future periods of time were intended to benefit the community by allotting specific lengths of time to tasks. Was this the beginning of the working week?
Sun Clocks
With the disappearance of any ancient civilization, such as the Sumerian culture, knowledge is also lost. Whilst we can only hypothesize on the reasons of why the equivalent to the modern wristwatch was never completed, we know that the ancient Egyptians were next to layout a system of dividing the day into parts, similar to hours.
"Obelisks" (tall four-sided tapered monuments) were carefully constructed and even purposefully geographically located around 3500 BC. A shadow was east as the Sun moved across the sky by the obelisk, which it appears was then marked out in sections, allowing people to clearly see the two halves of the day. Some of the sections have also been found to indicate the "year"s longest and shortest days, which it is thought were developments added later to allow identification of other important time subdivisions.
Another ancient Egyptian "shadow clock" or "sundial" has been discovered to have been in use around 1500 BC, which allowed the measuring of the passage of "hours". The sections were divided into ten parts, With two "twilight hours" indicated, occurring in the morning and the evening. For it to work successfully then at midday or noon, the device had to be turned 180 degrees to measure the afternoon hours.
Water Clocks
"Water clocks" were among the earliest time keeping devices that didn't use the observation of the celestial bodies to calculate the passage of time. The ancient Greeks, it is believed, began using water clocks around 325 BC. Most of these clocks were used to determine the hours of the night, but may have also been used during daylight. An inherent problem with the water clock was that they were not totally accurate, as the system of measurement was based on the flow of water either into, or out of, a container which had markers around the sides. Another very similar form. was that of a bowl that sank during a period as it was filled of water from a regulated flow. It is known that water clocks were common across the Middle East, and that these were still being used in North Africa during the early part of the twentieth-century.
Mechanical Clocks
In 1656, "Christian Huygens' (Dutch scientist), made the first "Pendulum(钟摆) clock", with a mechanism using a "natural" period of oscillation(振幅). "Galileo Galilei" is credited, in most historical books, for inventing the pendulum as early as 1582, but his design was not built before his death. Huygens' clock, when built, had an error of "less than only one minute a day". This was a massive leap in the development of maintaining accuracy, as this had previously never been achieved. Later refinements to the pendulum clock reduced this margin of error to "less than 10 seconds a day".
The mechanical clock continued to develop until they achieved an accuracy of "a hundredth-of- a-second a day", when the pendulum clock became the accepted standard in most astronomical observatories.
Quartz Clocks
The running of a "Quartz clock" is based on the piezoelectric property of the quartz crystal. When an electric field is applied to a quartz crystal, it actually changes the shape of the crystal itself, If you then squeeze it or bend it, an electric field is generated. When pla
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第1题
关于脱髓鞘疾病下列描述哪项是不正确的
A、脱髓鞘疾病是一组脑和脊髓以髓鞘破坏或髓鞘脱失病变为主要特征的疾病
B、中枢神经系统脱髓鞘疾病包括先天性和和后天性两大类
C、脱髓鞘疾病病理特征性表现是脱髓鞘,而神经细胞、轴突及支持组织保持相对完整
D、脱髓鞘病损分布于中枢神经系统白质
E、小静脉周围炎性细胞呈袖套状浸润
第2题
下列炎性脱髓鞘疾病的概念和分类哪项正确?()
A、炎性脱髓鞘疾病是指脑和脊髓以髓鞘破坏或脱髓鞘病变为主要特征的疾病,可分为多发性硬化、视神经脊髓炎、急性播散性脑脊髓炎、弥漫性硬化、同心圆性硬化、脑桥中央髓鞘溶解症
B、炎性脱髓鞘疾病是指以中枢神经系统白质脱髓鞘病变为特点的自身免疫病,可分为脑动脉炎、系统红斑狼疮、白塞病、颈椎病脊髓型、多发性硬化
C、炎性脱髓鞘疾病是指以中枢神经系统白质脱髓鞘病变为特点的自身免疫病,可分为多发性硬化、视神经脊髓炎、急性播散性脑脊髓炎、弥漫性硬化
D、炎性脱髓鞘疾病是指以中枢神经系统灰质脱髓鞘病变为特点的自身免疫病,可分为脑动脉炎、系统红斑狼疮、白塞病、颈椎病脊髓型、弥漫性硬化
E、炎性脱髓鞘疾病是指脊髓以髓鞘破坏或脱髓鞘病变为主要特征的疾病,可分为多发性硬化、视神经脊髓炎、急性播散性脑脊髓炎、弥漫性硬化、白塞病、同心圆性硬化、脑桥中央髓鞘溶解症
第3题
A、脱髓鞘疾病是一组脑和脊髓以髓鞘破坏或髓鞘脱失病变为主要特征的疾病
B、中枢神经系统脱髓鞘疾病包括先天性和后天性两大类
C、脱髓鞘疾病病理特征性表现是脱髓鞘,面神经细胞、轴突及支持组织保持相对完整
D、脱髓鞘病损分布于中枢神经系统白质
E、小静脉周围炎性细胞呈袖套状浸润
第4题
A、脱髓鞘疾病是一组脑和脊髓以髓鞘破坏或髓鞘脱失病变为主要特征的疾病
B、中枢神经系统脱髓鞘疾病包括先天性和和后天性两大类
C、脱髓鞘疾病病理特征性表现是脱髓鞘,面神经细胞、轴突及支持组织保持相对完整
D、脱髓鞘病损分布于中枢神经系统白质
E、小静脉周围炎性细胞呈袖套状浸润
第6题
下列疾病脱髓鞘病变不包括()。
A、肾上腺脑白质营养不良
B、中心性脑桥髓鞘溶解
C、急性播散性脑脊髓炎
D、多发性硬化
E、渗透性髓鞘溶解
第7题
下列疾病不属于脱髓鞘病变的是
A、中心性脑桥髓鞘溶解
B、多发性硬化
C、急性播散性脑脊髓炎
D、肾上腺脑白质营养不良
E、渗透性髓鞘溶解
第8题
下列不是脱髓鞘疾病的常见病理改变的是
A、神经纤维髓鞘破坏
B、病变分布于中枢神经系统白质
C、小静脉周围炎性细胞浸润
D、神经轴索严重坏死
E、神经细胞相对完整
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