全文获取类型
收费全文 | 20228篇 |
免费 | 243篇 |
国内免费 | 55篇 |
专业分类
教育 | 15038篇 |
科学研究 | 1399篇 |
各国文化 | 210篇 |
体育 | 1493篇 |
综合类 | 47篇 |
文化理论 | 104篇 |
信息传播 | 2235篇 |
出版年
2021年 | 203篇 |
2020年 | 247篇 |
2019年 | 384篇 |
2018年 | 553篇 |
2017年 | 596篇 |
2016年 | 529篇 |
2015年 | 361篇 |
2014年 | 552篇 |
2013年 | 4236篇 |
2012年 | 548篇 |
2011年 | 556篇 |
2010年 | 510篇 |
2009年 | 506篇 |
2008年 | 484篇 |
2007年 | 466篇 |
2006年 | 494篇 |
2005年 | 394篇 |
2004年 | 276篇 |
2003年 | 277篇 |
2002年 | 276篇 |
2001年 | 387篇 |
2000年 | 318篇 |
1999年 | 296篇 |
1998年 | 190篇 |
1997年 | 190篇 |
1996年 | 230篇 |
1995年 | 243篇 |
1994年 | 204篇 |
1993年 | 193篇 |
1992年 | 297篇 |
1991年 | 271篇 |
1990年 | 290篇 |
1989年 | 246篇 |
1988年 | 196篇 |
1987年 | 205篇 |
1986年 | 211篇 |
1985年 | 201篇 |
1984年 | 198篇 |
1983年 | 187篇 |
1982年 | 164篇 |
1981年 | 159篇 |
1980年 | 175篇 |
1979年 | 262篇 |
1978年 | 220篇 |
1977年 | 163篇 |
1976年 | 157篇 |
1975年 | 145篇 |
1974年 | 135篇 |
1973年 | 131篇 |
1971年 | 134篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
This paper summarizes some of the experimental evidence of the apparent biological effects produced by atmospheric ionization. Thermionic ion sources are discussed and the results of experimental determinations of their characteristics are presented. 相似文献
92.
93.
94.
Charles F. Kettering 《Journal of The Franklin Institute》1951,251(1):109-119
Nearly all prognostigators, from the fellow who buys a $2 ticket on Blue Boy to the financial policy-makers of government and business, base their final conclusions upon past experience. Properly tempered (and herein lies the success or failure of the prophet), this is a completely logical procedure, and is especially applicable to engineering trends, for most of our technical progress has been achieved through a process of evolution.Thus, one of the best ways to gain an insight as to the direction in which we're going is to note our present position and to review how we reached it.In the preceding discussion, I have tried to cover a few specific phases of our transportation future in just this fashion, in order to emphasize that we can expect tomorrow further applications and extensions of the things we are working with today. 相似文献
95.
96.
From the standpoint of circuit synthesis as well as for a fundamental comprehension of circuit behavior, it is desirable to be able to visualize the response of a circuit over an extended frequency range by means of a geometrical construction. Mathematically, the response of a circuit can be expressed with the La Place transform theory as a function of the complex frequency variable, p = α + jωw, in which a is the decrement and ω is the “real” angular frequency. Physically, the nature of the real-frequency response is usually not discernible by casual inspection of the mathematical function. This paper develops a means of visualizing this frequency response readily through the construction of a vector diagram from the response function. 相似文献
97.
98.
99.
100.
Ronald L. Ives 《Journal of The Franklin Institute》1946,242(4):243-277
Several arrangements of relay, condenser, and battery can be made to oscillate dependably. The electrical properties of these oscillators, as well as of a number of derivative devices, are here outlined. 相似文献