In a laboratory vault outside Paris is a small cylinder of platinum–iridium alloy that ___1___ the standard for all mass measurements worldwide. By an 1889 international accord, the mass of this metal cylinder defines the kilogram.

But that may soon change. The kilogram is the only unit of measurement still based on a ___2___. A second of time, for instance, is now defined in terms of an electron transition of the cesium atom. And ___3___. Those standards are universal and unchanging—unlike the official kilogram. The reference cylinder's mass has ___4___ through the years—not enough to throw off your bathroom scale, but enough to bother measurement scientists.

Some of them are meeting January 24th at the Royal Society in London to discuss future improvements to the measurement units. The plan is to eventually relate the kilogram to a universal number known as Planck's constant. But the technology needed to do that is not yet fully developed. So, for the time being, that little metal cylinder outside Paris will just have to ___5___. I mean, mass.
【视听版科学小组荣誉出品】
serves as man-made artifact the meter is tied to the speed of light drifted slightly keep pulling its weight
巴黎城外的一个玻璃罩里存放着一个铂铱合金圆柱体，这个圆柱体就 是全世界测量物体质量的参照标准。1889年某项国际协议中规定，该 铂铱合金圆柱体的质量为1千克。 但这个标准即将成为过去。千克是目前唯一一个以人工制品为参照标 准的计量单位。比如说，秒就是根据铯原子内部的电子跃迁时间来的 下定义，而米的定义则和光速有关。这些计量标准普遍通用、亘古不 变，而千克的参照标准则不然。历经多年，质量标准参照物——铂铱 合金圆柱体的质量已略有减轻。你倒不至于把浴室秤给扔掉，但计量 学家就有得操心了。 部分计量学家将于1月24日在英国皇家学会举行会议探讨计量标准的改 善问题，计划最终将千克这个计量单位与普朗克常数联系起来。然而 ，要完成这项工作，技术问题仍有待解决。因此，在此之前，那小小 的合金圆柱体还得继续扮演“重量”级的角色，当然，是在质量测量 方面的重量级角色。